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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Number of Securities, Continuous Unrealized Loss Position More Than Twelve Months

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Securities Financed Through Nonrecourse Resecuritization

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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