Notes Payable to Trusts - Schedule of Liabilities and Maximum Exposure to Loss Related to Trusts (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Notes payable to Trusts	$ 3,763,474,000
Operating Partnership
Variable Interest Entity [Line Items]
Unamortized debt issuance costs	(6,620,000)	$ (9,033,000)
Total notes payable to trusts, net	604,276,000	$ 610,314,000
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership
Variable Interest Entity [Line Items]
Investment Balance	3,590,000
Maximum exposure to loss	116,000,000
Difference	0
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership | Trust
Variable Interest Entity [Line Items]
Investment Balance	1,083,000
Maximum exposure to loss	35,000,000
Difference	0
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership | Trust II
Variable Interest Entity [Line Items]
Investment Balance	1,269,000
Maximum exposure to loss	41,000,000
Difference	0
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership | Trust III
Variable Interest Entity [Line Items]
Investment Balance	1,238,000
Maximum exposure to loss	40,000,000
Difference	0
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership | Notes payable to Trusts
Variable Interest Entity [Line Items]
Notes payable to Trusts	119,590,000
Unamortized debt issuance costs	(2,146,000)
Total notes payable to trusts, net	117,444,000
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership | Notes payable to Trusts | Trust
Variable Interest Entity [Line Items]
Notes payable to Trusts	36,083,000
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership | Notes payable to Trusts | Trust II
Variable Interest Entity [Line Items]
Notes payable to Trusts	42,269,000
Variable Interest Entity, Not Primary Beneficiary | Operating Partnership | Notes payable to Trusts | Trust III
Variable Interest Entity [Line Items]
Notes payable to Trusts	$ 41,238,000