Ideas are free and that's good news: you can talk about them, copy them and even transform them.
Of course you run the risk of being copied as much as you can copy others. So you'll make your idea public when you have more to win than you may lose:
If you're not sure where to start from
If you want to start building a user base to facilitate the launch of your product or service
If you're alone and need to build a team
If you need extra competencies to move faster
I believe all new ideas should be made public from the beginning... unless they're part of a greater strategy that has to remain secret.