Variable Interests - Schedule of Assets and Liabilities and Maximum Exposure to Loss Related to Trusts (Detail) - Operating Partnership [Member] - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Unamortized debt issuance costs	$ (7,223,000)	$ (9,033,000)
Net carrying amount of liability components	602,485,000	$ 610,314,000
Variable Interest Entity, Not Primary Beneficiary [Member]
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 [Member] | Notes Payable to Trusts [Member]
Variable Interest Entity [Line Items]
Notes payable to Trusts	119,590,000
Unamortized debt issuance costs	(2,176,000)
Net carrying amount of liability components	117,414,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Trust I [Member]
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 [Member] | Trust I [Member] | Notes Payable to Trusts [Member]
Variable Interest Entity [Line Items]
Notes payable to Trusts	36,083,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Trust II [Member]
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 [Member] | Trust II [Member] | Notes Payable to Trusts [Member]
Variable Interest Entity [Line Items]
Notes payable to Trusts	42,269,000
Variable Interest Entity, Not Primary Beneficiary [Member] | Trust III [Member]
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 [Member] | Trust III [Member] | Notes Payable to Trusts [Member]
Variable Interest Entity [Line Items]
Notes payable to Trusts	$ 41,238,000