Disc sighting in Detroit

This happened in August of 1980 about 11:30 at night in Detroit MI. Weather was partly cloudy.
Being summer, my father was sitting on the porch. He walked in and said, "Hey, do you want to see a UFO?"  
I thought, what a silly joke but got up anyway and went out on the porch. 
When I went on the porch I looked up and saw an enormous disc hovering in the sky above the trees to the east. It looked to be in the area of 300 feet long and had a row of illuminated portholes along the side.  It was silent and floating in the same manner as a boat would in water. 
My mother, grandmother, aunt and uncle also came out to look. We watched it a good 10 minutes before the tops of the trees obscured it.
Date is approximate.  PD