Of all the misconceptions people have about creative work, one of the largest — and one I find myself constantly having to counter — is the idea that whatever idea one has for a project, it should be jealously guarded like a trade secret until the work is done, because otherwise someone might steal it! Nothing could be further from the truth.
The first reason this pernicious myth persists is because of the other pernicious myth that it's ideas that make a story. Ideas are a necessary, but not sufficient, ingredient for a story — that is, you need an idea, to be sure, but you need plenty else on top of that. You need characters, setting, momentum, flavor — all the things that individuate a story and make it something not like the others. And more often than not, it's those things, not the underlying idea, that provides the individuation.
Ideas are cheap. In fact, many ideas are so cheap, they scarcely support a story on their own anyway. Some people compensate for this by making a story out of an amalgam of a whole slew of such ideas. Many of the recent maximalist SF projects I've seen lately do something like this: they don't start with a single what-if, but rather four or five of them smushed together. That makes it difficult to do justice to any one of those what-ifs, and so the results are often a gloppy stew instead of a blend. Pick one thing and run with it to the end of the pier, I say. (And then dive off into the lake with it.)
Jealously guarding one's ideas out of the mistaken notion that someone else will make more of them than we will does not make your idea any more original, or give you any more power to do justice to it. At least part of this seems to be due to a misinterpretation of the way big companies are notoriously secretive about the details of their projects. They have to be that way, because they have tons of money at stake, contracts to keep the secrets close, and lawyers to sue the ever-loving gravy out of people who break those contracts. You are almost certainly not in such a position of risk.
The other side of this is that it's impossible for anyone else to make of an idea what you will make of it. They will never bring the same perspective, the same additional details, the same way of fleshing-out, that you will. And those are all the things that make your story truly original anyway!
When I was younger and more attached to the idea of ideas, I played all of my work a lot closer to the vest than I do now. I'd say that I was working on something, but it wasn't until I was at the 99.5% mark that I would even start talking about what the damn thing was. Later I realized this was pointless; all it was doing was denying me of the freedom to share my ideas with people who might have useful things to say about them. (You don't have to take every piece of advice offered to you, either.)
What changed everything was when I decided that I would begin publicly documenting the unfolding of the idea, from the early stages on. That way, if someone blagged my idea, I had something in the public record indicating when I'd begun working on it. I wouldn't hide it, I'd provide it. And even if someone did blag the idea, so what? The odds of them ever coming up with anything remotely like what I was doing, or to the same ends, weren't even worth computing.
In the movie-adapted-from-the-book-of-the-same-name A Walk Among The Tombstones, one of the characters quips "People are afraid of all the wrong things." That sentiment is doubly true in creative work as it is in the rest of life. Don't worry about whether or not your ideas are likely to be stolen; worry instead about investing your incarnation of those ideas with the most of what you are. Besides, if someone has the gall to rip you off, that means you came up with something worth ripping off in the first place. Be proud.