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