Variable Interests - Schedule of Assets and Liabilities and Maximum Exposure to Loss Related to Trusts (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Unamortized debt issuance costs	$ (4,808,000)	$ (6,620,000)
Net carrying amount of liability components	560,613,000	$ 604,276,000
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Investment Balance	2,940,000
Maximum exposure to loss	95,000,000
Difference	0
Variable Interest Entity, Not Primary Beneficiary | Notes payable to Trusts
Variable Interest Entity [Line Items]
Notes payable to Trusts	97,940,000
Unamortized debt issuance costs	(2,053,000)
Net carrying amount of liability components	95,887,000
Variable Interest Entity, Not Primary Beneficiary | 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 | Trust | Notes payable to Trusts
Variable Interest Entity [Line Items]
Notes payable to Trusts	36,083,000
Variable Interest Entity, Not Primary Beneficiary | Trust II
Variable Interest Entity [Line Items]
Investment Balance	619,000
Maximum exposure to loss	20,000,000
Difference	0
Variable Interest Entity, Not Primary Beneficiary | Trust II | Notes payable to Trusts
Variable Interest Entity [Line Items]
Notes payable to Trusts	20,619,000
Variable Interest Entity, Not Primary Beneficiary | 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 | Trust III | Notes payable to Trusts
Variable Interest Entity [Line Items]
Notes payable to Trusts	$ 41,238,000