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Real Estate Securities - Additional Information (Detail) | 12 Months Ended
--- | ---
Dec. 31, 2015 USD ($) Investment | Dec. 31, 2014 USD ($) Investment
Investment Holdings [Line Items] |  |
Trading securities | $ 404,011,000 | $ 111,606,000
Trading securities acquired | 383,000,000 | 80,000,000
Trading securities sold | $ 18,000,000 | $ 62,000,000
Number of AFS securities | Investment | 224 | 290
Number of securities in unrealized loss position | Investment | 32 | 31
Number of securities in a continuous unrealized loss position for twelve consecutive months or longer | Investment | 15 | 10
Change in trading securities | $ (17,000,000) | $ (24,000,000)
Trading securities pledged as collateral | 347,000,000 |
AFS securities acquired | 33,000,000 | 237,000,000
AFS securities sold | 366,000,000 | 440,000,000
AFS net realized gains | 34,000,000 | 12,000,000
OTTI losses related to AFS securities | 1,000,000 |
Residential |  |
Investment Holdings [Line Items] |  |
Marketable securities, due from five to ten years | 0 |
Marketable securities, less than five years | 2,000,000 |
Securities pledged as collateral | 827,000,000 | 762,000,000
Interest Only Securities |  |
Investment Holdings [Line Items] |  |
Trading securities | 37,000,000 |
Senior Securities |  |
Investment Holdings [Line Items] |  |
Trading securities | 254,351,000 | 101,753,000
Senior Securities | Prime |  |
Investment Holdings [Line Items] |  |
Trading securities | 248,570,000 | 93,802,000
Securities financed through nonrecourse resecuritization |  | 105,000,000
Senior Securities | Non-prime |  |
Investment Holdings [Line Items] |  |
Trading securities | 5,781,000 | 7,951,000
Securities financed through nonrecourse resecuritization |  | 117,000,000
Residential Subordinate Securities |  |
Investment Holdings [Line Items] |  |
Trading securities | 149,000,000 |
Residential Subordinate Securities | Trading Securities |  |
Investment Holdings [Line Items] |  |
Unpaid principal balance | 168,000,000 |
Residential Senior and Subordinate Securities |  |
Investment Holdings [Line Items] |  |
Trading securities | 218,000,000 |
Residential Senior and Subordinate Securities | Trading Securities |  |
Investment Holdings [Line Items] |  |
Unpaid principal balance | 217,000,000 |
Available-for-sale Securities | Residential |  |
Investment Holdings [Line Items] |  |
Securities pledged as collateral | 480,000,000 |
Residential |  |
Investment Holdings [Line Items] |  |
Gross unrealized losses | 5,007,000 | 2,796,000
Residential | Senior Securities | Prime |  |
Investment Holdings [Line Items] |  |
Gross unrealized losses | 3,491,000 | 1,298,000
Residential | Senior Securities | Non-prime |  |
Investment Holdings [Line Items] |  |
Gross unrealized losses | $ 86,000 | $ 61,000

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