This is a nice small article on why you should share your game designs.
Moral #1: The final game is not going to look anything like your initial game design because ultimately it is the game director who makes the most important decisions, not the person who writes the game design document.
Moral #2: Most people like to copy successful ideas. Original ideas are far less likely to be cloned because they are seen as risky.
Moral #3: Most people are absolutely horrible game designers. Your game design could probably be dramatically improved by talking to other skilled designers. You have dramatically more to gain by sharing than by hording.
Moral #4: If your design ideas are similar to another title, there is a good chance you are both cribbing from the same cheat sheet. Relax. Your super clone isnt going to win or lose based off the game design anyway. Brand, polish and production values are more important.
If you do share your game design, I offer you this prediction:
No one will steal your game design.
By sharing your game design with other competant designers, you will receive in return invaluable feedback that improve your final game.
You will contribute to the game development community and help others learn about game design.