We are not lawyers, and the answer to your question will vary from location to location (even state to state in the US). If you want legal advice, your best bet is to consult an IP lawyer.
This is exactly my advice to you if you think you have an idea that is worth getting worried about potential theft. Consult an IP lawyer, apply for a patent, and keep the idea on a “need to know” basis. While there are certainly those out there who would steal an idea, in my experience they are few and far between. What’s more, it’s one thing to steal an idea, and another thing entirely to execute it. The ability to obtain a concept and the ability to put it into practice are two completely different skill sets. I’m not sure that you have a ton to be worried about.