Basis of Presentation - Statement of Beneficiaries' Equity (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	$ 1,702,437	$ 1,729,665	$ 1,760,463	$ 1,811,028	$ 1,756,294	$ 1,830,525	$ 1,847,251	$ 1,826,870
Additional Paid-in Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	3,189,350	3,188,239	3,186,774	3,200,312	3,223,318	3,222,585	3,221,560	3,218,077
Cumulative Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	787,789	781,017	774,812	775,625	654,744	697,916	685,166	641,093
Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity	$ 11,695	$ 11,846	12,025	12,201	16,669	17,246	17,375	17,258
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity				1,832,573
As Previously Reported | Additional Paid-in Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity			3,187,312	3,200,850	3,223,817	3,223,072	3,222,047	3,218,564
As Previously Reported | Cumulative Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity			795,186	796,513	674,599	717,515	704,506	660,174
As Previously Reported | Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity			12,142	12,320	16,824	17,410	17,538	17,420
Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity				(21,545)
Adjustments | Additional Paid-in Capital
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity			(538)	(538)	(499)	(487)	(487)	(487)
Adjustments | Cumulative Earnings
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity			(20,374)	(20,888)	(19,855)	(19,599)	(19,340)	(19,081)
Adjustments | Noncontrolling Interests
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Stockholders' equity			$ (117)	$ (119)	$ (155)	$ (164)	$ (163)	$ (162)