There's literature to help you with organizing your thoughts. You've read some books on game design, I suppose? The one I'd recommend is (here we go again)
Game Design: Theory and Practice (2nd Edition), especially since one of its chapters deals precisely with writing a design document and there are even two sample documents at the end of the book. It's my favorite book on game design out of 3 or 4 I've read so far - some of them claimed they were on game design, but were mostly about level design or game development in general.
And yeah, I'd like to hear about your idea. I wouldn't mind sharing some of mine, either, if you're interested.