Besides, ideas grow and take on a life of their own as you work on them. By the time your interpretation of the idea takes shape it's going to be distinctly different from the original idea that inspired you.
The only way to keep people from stealing your ideas is by never talking to anybody about them. That's surely the worst of all options.
You can literally try to clone any YC company after demo day. All the startups get covered on techcrunch making their ideas public. Just pick the one you like best and open your favorite editor. They only got a 6 months head start, so what's stopping you?