People who startup companies are cautious. I have seen numerous websites say "We are in stealth mode and we will be up and running in a couple of months" which I interpret as "We are scared to share our idea because we think you will steal it and launch a company before we do". Is it really that easy to steal an idea ?
I don't think so. If an idea were so valuable, wouldn't there be a market for ideas ? If you want to find out how valuable an idea is, try selling one in the market and you will find out the reality. What is really valuable though is an idea that has been implemented in the form of a product, and you can certainly sell that for a couple million dollars.
Another way to think about it is to take an example, say search engines. The idea of search engines is really old, but over the years, many have come and gone. When google launched their search engine, there was nothing unique in the idea, what was unique was their approach to the problem and the implementation. That was what eventually took them to their current multi-billion dollar valuation and the greenbacks in the pockets of Sergey and Larry.
So think twice before you hide an idea - discussing it with someone might give you more value than hiding it inside your head.