Captain reported APU failure in flight with the #2 IDG on MEL. The Flight Crew elected to request priority handling and diverted to make a precautionary landing.

Narrative

The left engine IDG was operational but the right engine IDG was deferred. Per the MEL; the APU was to remain operational throughout the entire flight to provide an additional AC power source. About 3-5 minutes after leveling off at our final cruise altitude of FL320; an EICAS caution message appeared indicating an APU Auto Shutdown. The airplane was now down to one main AC generator. We requested priority handling. Our position was approximately 80-85NM east of ZZZ. We diverted to ZZZ. Throughout the event; the airplane remained stable and operated; otherwise; normally allowing us to land safely and uneventfully. The Captain and myself; the First Officer were involved.It is important to note that we had flown the airplane on 3 previous flights that day. During the first flight from ZZZ1-ZZZ and after level off at our final cruise altitude; the Captain performed a routine system checks using the EICAS System Display. The electrical page indicated the number 2 engines IDG frequency fluctuating between green/amber and below 400hz; between 391-390hz. Upon arriving in ZZZ; the aircraft was grounded by Maintenance for about 2 hours and the engine 2 IDG was deferred. Per the MEL the APU was required to remain operational throughout the flight. During the next two flights; the airplane operated normally and we had no further indications of any electrical problems. We also had no indications of APU problems. The main casual factor in this event was the APU automatically shutting down (the EICAS did not indicate why the APU ECU shut it down) causing us to only have one main AC power source remaining due to the IDG 2 deferral.As a pilot; I do not claim to understand or know the operational complexities of an airlines flight operation and related maintenance decisions. It is evident; by an existing MEL; that it is acceptable to operate an aircraft for up to 3 days (category B repair) with an engine IDG inoperative as long as the APU remains operational. So my suggestion to prevent this type of event occurring in the future is simply that if the aircraft had been taken out of service; the IDG 2 repaired or replaced; it would or may have prevented this issue.