I have a poster session tomorrow about an idea I didn't publish yet but planning very soon. If people stop by and asked me in detail about my idea, should I openly discuss or not?
I certainly would. It is often very beneficial to discuss your work with somebody who can understand it. You may get some interesting remarks, suggestions that you didn't think of, or just some information that you are not aware of. Besides, the openly exhibited attitude of "I'm not telling you anything of value because I suspect you are a petty thief trying to steal my intellectual property" leaves a very unpleasant impression and the evasive moves of the sort "This would take too long time to explain" may just make people suspect that you don't know what you are talking about yourself. So if you do not want to share your ideas at the moment for whatever reason, I would just not go public with anything at all.
With that said, the chance that somebody will stop by and engage in a long meaningful discussion with you during a poster session is not too high (unless you present something really outstanding), and you certainly do not need and aren't expected to tell everything you can in the case of the usual "small talk".
Just my two cents.