Variable Interest Entities - Summary of Assets and Liabilities of Consolidated Securitization VIE (Detail) $ in Thousands	Sep. 30, 2017 USD ($)
Assets:
Assets	$ 500,800
VIE [Member]
Assets:
Assets	500,763
Liabilities:
Liabilities	474,902
VIE [Member] \| Loans Receivable [Member]
Assets:
Assets	500,000
VIE [Member] \| Other Assets [Member]
Assets:
Assets	763
VIE [Member] \| Securitized Debt Obligations, Net [Member]
Liabilities:
Liabilities	474,298
VIE [Member] \| Other Liabilities [Member]
Liabilities:
Liabilities	$ 604

Amount of consolidated variable interest entity's (VIE) liabilities that do not have recourse to general credit of primary beneficiary.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=77990344&loc=d3e5710-111685

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6839525&loc=SL6758485-165988

Name:	us-gaap_VariableInterestEntityConsolidatedLiabilitiesNoRecourse
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant