Okay, technically speaking, you can copyright an idea. You don't even have to register it with the government to claim that it's yours (but you should, 'cause any legal problems will be a lot easier to handle). However, you gotta put it down on paper and date it if it's gonna hold any water whatsoever. Plus, you really have to prove you submitted that idea to the person who allegedly stole it. And even if you don't submit it, if it sits on a shelf gathering dust and someone else comes along with the same idea, well, your outta luck there, too.
See, I could come up with the sweetest idea for a movie right now and furiously scribble it down on a notepad. Once I do that, I have an intellectual copyright on it. But if I sit on it and don't let anyone else see it and Pixar comes out with a movie just like it four years from now, I'm up a creek. I didn't let 'em see it, so how can I claim that they stole it? The court would tell me "Sorry 'bout your luck", and move on. Now, if I had sent it to John Lassetter back in '98, got a rejection letter back, and then the same movie came out eight years later, I may have a case.