Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Outstanding letters of credit and performance bonds	$ 2,600
Future obligations under leases				$ 100,200
Lease operating expenses	3,600	$ 3,300	$ 2,400
Other Commitments [Line Items]
Operating leases	$ 205,643
Weighted average remaining term (in years)	2 years
Ground Leases
Other Commitments [Line Items]
Weighted average remaining term (in years)	76 years