Instagram is a good example of this. "A mobile app where people can share photos" is not a very valuable idea. "A mobile app where people can add filters to imbue meaning into photos, combined with a quick and beautiful interface, combined with a social stream, commenting, favoriting, and notifications, packaged into a easy to use app" turned out to be more valuable. Even more valuable is that idea combined with a number of sketches, designs, user surveys, and proofs of concepts. These levels of detail all fall under the umbrella of "idea" -- but it's understanding the core thesis of an idea combined with being able to see it's consequences in the details that can actually be valuable.
This means sharing your idea can be dangerous if a) you are sharing it at a sufficient level of detail and b) you are sharing it with someone who can empathize with the thesis of the idea and draw the same logical conclusions in terms of implementation details. Basically, if someone "gets it" and can execute on that understanding.