Air carrier flight crew performed a rejected takeoff due to generator drive EICAS message.

Narrative

I was the pilot flying (PF). During the takeoff roll; the Captain announced he is rejecting the takeoff. It was due to the illumination of the L GEN DRIVE EICAS message. We had positive exchange of the flight controls. I backed him up on his procedure and when I looked at the airspeed indicator we were at approximately 85 kts. The auto brakes kicked in.I notified ATC we were rejecting on Runway XXR. After making sure everything was under control I made the remain seated; remain seated;" passenger PA. We taxied off 1 intersection and we monitored the brake temperature. By this time the EICAS message extinguished.I completed the after takeoff flow; the Rejected Takeoff QRC; Rejected Takeoff QRH; and continued to monitor the brake temperature. I coordinated with operations for a new gate and Maintenance; Tower and Ground for taxi instructions; and ramp to block in.After block in I called the chief pilot to notify him of the rejected takeoff while the Captain coordinated with Dispatch and the Gate Agent. Maintenance discovered the Integrated Drive Generator (IDG) service cap was off and the left generator had no more oil. They serviced it and did an engine run up at the gate. The generator operated normally after getting serviced."