an IDG is an Integrated Drive Generator,
Correct. It is a combination of a constant speed drive (CSD) and a generator in one housing. A CSD is a hydromechanical transmission that takes a variable input speed and through some trickery turns the generator input shaft at a constant speed.
The Boeing 787 combines this generator role
The 787 doesn't have IDGs (or CSDs). The engine gearbox drives the generators at variable speeds, producing variable output frequencies (as opposed to a steady 400 Hz). Power electronics is used to convert this variable frequency power into 400 Hz (or whatever the loads require).