B757 First Officer reported hydraulic system malfunction during cruise. Flight diverted and landed safely.

Narrative

At FL360 in cruise flight approximately 200 miles west of ZZZZ we received an EICAS message L HYQ QTY. We directed our attention to the STATUS page and observed the LEFT hydraulic quantity at 0.45 and continuing to decrease. The CA began running the appropriate QRH checklist while I continued to fly the airplane and communicate with ATC. The QRH directed us to turn the electric and engine driven pumps off to the affected side to save fluid. This was accomplished but fluid continued to be depleted down to about 0.18. The captain and I agreed to request a turn back to ZZZZ as it was our nearest airport but also the most suitable for this type of non-normal. We requested priority handing with ATC and communicated with the Flight Attendants; Dispatch and Maintenance Control. We set up for the ILS XX at ZZZZ and requested our landing performance from dispatch as well as checking it ourselves. We received clearance to 3;000 feet and subsequently cleared the ILS approach. 10 miles from ZZZZZ the IAF we elected to bring the gear down early. The hydraulic pumps were turned back on per the L HYD QTY checklist. The gear came down when selected down but most of the remaining hydraulic fluid exited the Left system at that time and the gear doors were unable to come back up into the closed position. We received a L HYD SYS PRESS EICAS. We briefed that this may happen so we completed the applicable QRH which called for alternate flap extension and flaps 20 landing. This was completed without issue and the approach was flown with flaps 20 at an approximate ref of 160 KIAS. A overweight landing at 220.0lbs was accomplished. Touchdown was normal and we rolled to the end of RWY XX and the hydraulic fluid was nearly gone from the left side but we still had nose wheel steering available from the help of the PTU (Power Transfer Unit). We were able to taxi do our stand under our own power until the last 20 feet when the PTU said no more and we received a STATUS message. Parking was accomplished with just a couple pushes on the rudder pedals for the last 10 feet.