While I know cases of stealing by people who heard the idea, or even unintentional incorporation into their other work (one often does not remember which idea helped her/him to come to certain point), this is possible in an unwanted way ONLY if this passing of idea is individual and in a way secretful. If you tell the idea to LOTS of people, at recordable places as well, then everybody knows it is your idea and you will get credit and be allowed to publish it even if some use the idea in various forms. So making it half-available is the only scenario which can possibly hurt you (though such cases are rare in pure mathematics and somewhat more common in areas with more money like physics, biomedical sciences and so on). There is no better protection from stealing a secret than making it no-secret at all.