Commitments and Contingencies - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Ground rent expenses	$ 18,499,000	$ 16,495,000	$ 17,938,000
Contingent liability under letters of credit	60,588,000
Outstanding obligations under construction agreements	40,121,000
Contingent rent incurred	$ 0	$ 0	$ 0