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Consolidated K-Series - Narrative (Details) $ in Millions | Sep. 30, 2017 USD ($) securitization | Dec. 31, 2016 USD ($) securitization | Dec. 31, 2015 securitization | Dec. 31, 2006 securitization
--- | --- | --- | --- | ---
Multi-family collateralized debt obligations, at fair value |  |  |  |
Mortgage Loans on Real Estate [Line Items] |  |  |  |
Loans, unpaid principal balance | $ 8,100.0 | $ 6,700.0 |  |
Weighted average interest rate | 4.07% | 3.97% |  |
Multi-family loans held in securitization trusts |  |  |  |
Mortgage Loans on Real Estate [Line Items] |  |  |  |
Loans, unpaid principal balance | $ 8,100.0 | $ 6,700.0 |  |
Consolidated K-Series |  |  |  |
Mortgage Loans on Real Estate [Line Items] |  |  |  |
K-Series, carrying value | $ 408.7 | $ 314.9 |  |
Consolidated K-Series | K-Series |  |  |  |
Mortgage Loans on Real Estate [Line Items] |  |  |  |
Number of securitizations | securitization | 6 | 5 | 4 | 4

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- Definition Number of Securitizations + References No definition available. + Details
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- Definition Total carrying amount of mortgage loans as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 10 -Section S99 -Paragraph 6 -Subparagraph (SX 210.5-04.(c) Schedule IV) -URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 310 -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-29.6) -URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014 + Details
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