Variable Interests - Schedule of Assets and Liabilities and Maximum Exposure to Loss Related to Trusts (Detail) - Operating Partnership - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Unamortized debt issuance costs	$ (5,407,000)	$ (6,620,000)
Net carrying amount of liability components	570,425,000	$ 604,276,000
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	(2,084,000)
Net carrying amount of liability components	117,506,000
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