Ask HN: Legalities of developing "worthless" ideas on company time?

Does the law agree that ideas are worthless? Suppose while working for a company I have an idea for a great product. I quit and build the product - what does the company own, if anything?

What if I made drawings of the idea, or created a business plan before quitting?

I mean not actually on "company time" (while on premise between 9 to 5", but say, in the evening. But some companies have these contracts that they own everything you come up with while you work for them.