I've had the same questioning once, when I've met my co-founder at Wideo.co
He told me something I always remember. If you have a great idea, you were probably have been thinking about it and every detail at least some months ago. So what you can tell in a fraction of time to someone to give a brief idea, won't ever describe the amount of information, specificity, sub ideas, contacts, business model, and every minor detail you've been planing since then.
So don't be afraid to share your idea since no one would be able to execute as well as you, if you really have something powerful and value in your mind.
What's more, speak about it with every person you met, and get as much feedback as you can (the sooner, the better)

I hope you find my advice useful.
Good luck with the project!