Roll-Forwards of Allowance for Loan Losses for Commercial Mortgage Loans, Mezzanine Loans and Preferred Equity Interests (Parenthetical) (Detail) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($)
Receivables [Abstract]
Impairments related to loans transferred to held for sale	$ 3,908

Value of loans and leases held in the entity's portfolio transferred to loans and leases held-for-sale in noncash transactions.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4332-108586

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4313-108586

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration