Organization and Basis of Presentation - Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2019
Ground Leases Disclosures [Abstract]
2019 (remainder of year)		$ 373
2020		1,370
2021		1,234
2022		1,136
2023		1,035
Thereafter		40,684
Total minimum payments		45,832
Imputed interest		(26,941)
Amortization		(512)
Lease liabilities	$ 0	18,379
Deferred rent		(715)
Right of use asset	0	17,664
Leases, Operating [Abstract]
2019	1,512
2020	1,356
2021	1,205
2022	1,082
2023	981
Thereafter	40,799
Future minimum payments due	46,935
Leasing expense	$ 1,500
Operating Leases Receivable
Remainder of 2019		220,242
2020		500,908
2021		452,679
2022		382,294
2023		314,211
2024		259,139
Thereafter		977,267
Total		$ 3,106,740