How much is an "idea" worth, if I thought it up while working at my company? What happens if I leave now and start a new company with the idea? I haven't spent any time actually "implementing", and it's simply at the "idea" stage.

Like many people will answer: an idea alone is not enough to be worth something. It is the application and proof of concept that can turn an idea into a valuable commodity and company. Because starting companies have to be evaluated somehow, we look at the potential return on investment as a way of telling us if something is a good "idea" and if it is "worth" something. An idea that you believe in can be worth quite a bit, but mostly just to yourself. Since your context is to take that idea from theory and into reality I'd direct you towards a few articles from Venture Hacks on Ideas: http://venturehacks.com/archives#ideas You can find some great information on how to execute your idea (and perhaps reveal it's worth). And since you mentioned that you would like to leave your current company and start implementing your idea I'd advise that you try first to implement it on the side. Take your spare time (if the idea is actually something you believe in than you can start showing that by sacrificing your free time) and attempt to build it out. This is a good indicator to see if you are really prepared to make the leap and fully commit to making your idea. But jumping from your current company might be a little risky (unless you have enough financial backing to afford it), whereas spending nights and weekends to prototype it can be low-risk and beneficial.

You keep putting idea in quotation marks... which makes it hard to answer this. But ideas are worth something.  That's called "Intellectual Property" and that's why we have things like trademarks, patents, and copyrights.  Those all protect "ideas" someone has.  There's a whole area of law around Intellectual Property and lawyers who make a lot of money because it's not as simple as asking a question on Quora to find out what an "idea" is worth. But as everyone else here has said - ideas don't have value until they are implemented in some way.  That doesn't necessarily mean by you.  If you filed a patent on something, but didn't put it into production and someone else wanted to? That idea might have some sort of monetary value that you and they agreed to. However: a caveat - ideas seldom happen in a vaccuum.  If you go out and build the "next big thing" and it ends up being financially successful? Your current employer is probably going to try and find a way to argue that without working for them, you would not have thought of it.  (Think Arthur Fry who wouldn't have come up with Post-It notes if Spencer Silver hadn't already come up with the adhesive & he wasn't working at 3M.  If he had quit 3M with an "idea" to use their adhesive in a way they hadn't been trying to use it - you can bet 3M would've won that lawsuit.) That's why people are talking about IP agreements and Non-competes and NDAs.  It's also why you should have a lawyer look at anything you sign before signing it.

Most likely, the "idea" alone is not worth much.  Developing it, building a business which the idea inspires is worth money.  If you decide to start a company, if you get some qualified advice that this is the best thing to do, then the sooner you cut the umbilical cord with your present company the better.

It is common for employers to require its employees to sign an agreement  that may have transferred ownership of any patentable or other intellectual property you developed in the course of your employment, particularly if related to your job duties. That said, if all you are talking about is an inchoate "idea" that doesn't have any tangible proof of existence (outside your head) your employer will have a tough time proving a case against you. If the "idea" ends up being extremely valuable (we've all seen "The Social Network") then your employer will have a greater incentive to pursue a remedy against you. You should first determine if you signed any kind of "work made for hire" or other agreement with your employer governing ownership of intellectual property developed on the job.

If you actually want to risk your job for this idea and have NO regrets in the end, you at least take some time to find out whether it is worth risking your job for. To be worth anything, your idea should be for a product that solves a problem for a certain group of people. And it solves this problem better than the products these people are currently using to solve this problem. You cant just guess here, you need to go to the people who you think have this problem and first find out if they truly have the problem you think they have, and they care enough about it to pay for a solution. Then you present your solution to them and ask if they feel it solves their problem better than the product that they currently use to solve this problem. If it doesn't then you need to find out how you can do this. Only when you are sure that you  have a solution, to a problem that enough people, care enough about, to buy a solution for, only then can you quit our job and start building; in fact, i'll even advice to finish building and get some traction before you quit. This will help you avoid building something nobody wants, which is incidentally what kills most startups. I always advice my clients to always start with a problem and then build a solution for that. Most startups start with "clever ideas" and thats why most startups fail.

The idea is not worth much at all, if anything. The only time an idea is valuable is once it has been executed. Ideas are a dime a dozen, execution is the difficult part. You should check with a few different lawyers about whether you can implement the idea. I don't know if you signed a non-compete agreement or an NDA when you were hired, but you should make sure.

If you take your idea and make it successful, only then you will have to worry if your current company will come after you.  You do say it us not related to your current work, so they do have a tough case to prove. You have a lot more challenges before you reach the point when that (if) occurs.

Unfortunately, ideas generally worth nothing... But in this case, it could be your saviour since you apparently haven't started implementing or executing anything, not even a business plan. Importantly, beside the nature of the contract you signed when you joined the firm in question and the legal framework around IP and patent in the country/state you live in, the key element in your questions seems to be that your idea is not at the centre of your employer's core competences or capabilities. If, say, you work for a shampoo manufacturer, and you end up having an idea for a new type of... shampoos or any related products, then you are likely to be in trouble if you go away and set up your own business. The employer could argue that it was part of your responsibilities to be innovative in the very business sector you were working in. The other key factor in your questions is that your idea was not derived by any patent or process that would be the sole property of your employer. In short, from a moral and business standpoint, my guess is that you are fine. But I would still double check with an IP lawyer, just in case...