You cannot successfully protect your idea, and you don't want to.
Once you start your business, the idea will be out there. Others will copy it if it looks like the idea might be successful. In fact, it wouldn't be surprising if somebody else is working on a similar idea--- a lot of things happen that way when the time is right. Being one of several people with a similar idea can actually help give credibility to what you're trying to build. If you're too far out there, investors and employees will probably not want to work for you.
The biggest danger to your company is not that somebody steals your idea and makes it successful. After all, that doesn't mean you won't be successful *too*. The biggest danger is that the big idea simply doesn't work. The way you mitigate that risk is by validating the idea with real customers. (And, secondarily, partners and investors.) You can't do that without getting your idea out in the open.
Nobody can tell if your idea will work in isolation. "Nobody knows anything!" is a quote with a lot of truth. The people you talk to can't be certain that the idea is even worth stealing, if they wanted to! Only by testing it in the marketplace will the truth emerge.