How do I legally document an invention idea before I go forward with it and share the idea with an invention company?

The best thing for you to do is to file what is call a provisional patent application with the US Patent and Trademark Office. It is a low cost filing that need not be in a particular format and you can find all the information you need about it on the uspto.gov/patents/resources/types/provapp.jsp web page.  Separately, I caution you about using an invention company without first finding out their success rate, for example, how many of their clients have obtained more as a result of the company's services than they paid the company in fees, and without first checking to insure that it is not one of the invention companies that legal action has been taken against by client, a state's attorney general or the USPTO.