An invention can be patented, but an idea cannot. To learn all the rules and regulations, visit www.uspto.gov, which is the site for the US Patent and Trademark office. It can be challenging to protect an idea. I suggest you meet with an attorney to see if a non-disclosure agreement would be of benefit to you.
Creating a MAP will take no more than an hour of your time every month and will keep the lines of communication open, ensuring relationships with investors remain strong, and ultimately helping early-stage startups succeed.