Use of Special Purpose Entities (SPE) and Variable Interest Entities (VIE) - Key Details of Financing VIEs (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) securitization
Variable Interest Entity [Line Items]
Securitized debt, carrying value	$ 81,537	$ 158,867
Multi-family CMBS Re-securitization
Variable Interest Entity [Line Items]
Securitized debt, principal amount	33,350	33,553
Securitized debt, carrying value	$ 29,164	28,332
Multi-family CMBS Re-securitization \| LIBOR
Variable Interest Entity [Line Items]
Interest rate (as a percent)	5.35%
Distressed residential mortgage loans
Variable Interest Entity [Line Items]
Securitized debt, principal amount	$ 53,089	132,319
Securitized debt, carrying value	$ 52,373	130,535
Distressed residential mortgage loans \| LIBOR
Variable Interest Entity [Line Items]
Interest rate (as a percent)	4.00%
Distressed residential mortgage loans held in securitization trusts (net) \| Distressed residential mortgage loans
Variable Interest Entity [Line Items]
Residential mortgage loans held in securitization	$ 121,791	$ 195,347
K-Series \| Multi-family CMBS Re-securitization
Variable Interest Entity [Line Items]
Number of securitizations \| securitization		2

Number of Securitizations

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- Definition

The value of the collateral (typically cash) that the entity receives in a financing transaction in exchange for securities lent. This item represents a liability in the financial statements of the lender of the securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i)(A)(ii))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 25
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64840732&loc=d3e113275-111724

+ Details

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- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

+ Details

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- Definition

This is the principal amount outstanding for securitized loans only (across all types of loans).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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- Details

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- Details

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