Christine is correct. At the time, exercise was really just ‘aerobic dance’, and that’s how IDEA came to be an acronym. Over the years, IDEA has morphed well beyond that, but the name has stuck.
And IDEA has been around for as long as I can remember. It is one of the original fitness industry organizations. I have been seeking out fitness information since the mid 70s, and ACSM is the only other organization that I recall from further back than IDEA. (Yes, I am older, but pride myself on my immaturity when it comes to playing for fitness and fun.) This is one of the reasons that I am an IDEA member. We should all support a few of our favorite fitness organizations financial by becoming members. The work these groups do everyday supports us all.