I am sure that you have it written out in detail if you are to the point that you want to share it. If not, get to putting it down on paper. Make sure you are VERY detailed in it. Make sure you put the date on the paper (all pages), put it in an envelope, seal it and put some sort of stamp across the seal (so that if it was opened you or someone else would be able to tell), and mail it to yourself. DO NOT EVER OPEN IT! This is your proof that it was your idea and the point is that the envelope is only opened if needed.
My father told me about this a while back when I had an idea (I never acted on it though). He said it is just like having a patent, but without the legal stuff. If someone takes your idea after you have presented it to them without compensation to you, then you have proof that it was your idea. I would also include what your intentions are with the idea (i.e., planning to contact Company X to discuss in April or what progress you have made to date in testing, etc.). It is suppose to be very detailed.
Good luck!