Variable Interest Entities - Summary of Assets and Liabilities of Consolidated CLO and 2017 Single Asset Securitization VIE (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Assets	$ 1,500,000	$ 1,500,000
VIE [Member]
Assets:
Assets	1,504,899	1,502,407
Liabilities:
Liabilities	1,286,231	1,283,791
VIE [Member] | Loans Receivable [Member]
Assets:
Assets	1,500,000	1,500,000
VIE [Member] | Other Assets [Member]
Assets:
Assets	4,899	2,407
VIE [Member] | Securitized Debt Obligations, Net [Member]
Liabilities:
Liabilities	1,284,362	1,282,412
VIE [Member] | Other Liabilities [Member]
Liabilities:
Liabilities	$ 1,869	$ 1,379