I used to have the same fear, and it held me back for many years from taking any real action in my life. What I've discovered is that ideas are worthless without the ability to execute on them, and that until I started talking to people about them, I could never execute on them in any real way.
Moreover, if an idea is not my own, then my ability or interest in seeing it through to fruition fall somewhere between slim and none. Companies have their own initiatives and projects: they most likely won't have interest or capacity to take on yours. On the contrary, they may end up partnering with you to develop it. Brett Kelly's "Evernote Essentials" is a great example of that.
Even with all that, I will admit there is a slight risk that an unethical company will take your idea and run with it. If that's the case, and you can't proceed without cluing them into it, then you need to decide whether that's a risk worth taking. If you don't, your idea is worthless anyway, so what do you really have to lose? If they do steal it, then it's probably not a company you should waste time developing a product for anyway. Either way you connected your insight to action, which will undoubtedly lead you to more insight and more action: Win-Win!
I hope that perspective quiets your fears and empowers you to step boldly in your life so you can start exercising your genius in financially substantive ways. If you feel you could benefit from some additional support or clarity, I would welcome the opportunity to speak with you. All my client conversations are confidential.