Ideas are dime a dozen. Also, the vast majority of ideas people think are good are either unworkable or unmarketable. If your idea is the kind that you can build quickly and with the help of a single developer, it is almost certainly not worth stealing. If it is a serious, well-thought out proposal, then you'll probably need a bunch of engineers and the venture for a startup. Either way, you can always find a couple of computer science students and explore the outline of the idea with them.
But like I said, ideas are cheap. I come up with ideas all the time. Just a few month ago, I built a prototype shoes that can warm up on demand without the need to ever charge them. It was a fun project, but taking the next step requires too much effort. If someone wants to steal this idea, btw, I'd be more than happy to provide you with all the details.