Borrowings - Repurchase Agreements (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	$ 14,723,223		$ 15,451,675
Percent of Total Amount Outstanding	100.00%		100.00%
Company MBS Held as Collateral	16,455,482		17,079,377
Credit Suisse Securities (USA) LLC
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	1,635,598		1,809,896
Percent of Total Amount Outstanding	11.00%		11.80%
Company MBS Held as Collateral	2,045,230	[1]	2,203,883	[2]
South Street Securities Llc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	1,058,257		1,236,812
Percent of Total Amount Outstanding	7.20%		8.00%
Company MBS Held as Collateral	1,113,213		1,286,384
Banc Of America Securities Llc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	951,719		1,043,689
Percent of Total Amount Outstanding	6.50%		6.80%
Company MBS Held as Collateral	1,038,176		1,146,151
Citigroup Global Markets Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	903,037		1,027,210
Percent of Total Amount Outstanding	6.10%		6.60%
Company MBS Held as Collateral	1,049,651		1,164,162
Wells Fargo Securities, LLC
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	888,022		857,824
Percent of Total Amount Outstanding	6.00%		5.60%
Company MBS Held as Collateral	1,060,729		996,151
HSBC Securities (USA) Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	811,137		787,462
Percent of Total Amount Outstanding	5.50%		5.10%
Company MBS Held as Collateral	836,622		809,230
J.P. Morgan Securities Inc.
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	758,758		875,201
Percent of Total Amount Outstanding	5.20%		5.70%
Company MBS Held as Collateral	876,577		1,001,116
Pierpont Securities L L C
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	696,734		791,572
Percent of Total Amount Outstanding	4.70%		5.10%
Company MBS Held as Collateral	725,832		824,184
Ing Financial Market Llc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	680,130		676,644
Percent of Total Amount Outstanding	4.60%		4.40%
Company MBS Held as Collateral	723,067		718,086
Industrial And Commercial Bank Of China Financial Services Llc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	668,172		493,906
Percent of Total Amount Outstanding	4.50%		3.20%
Company MBS Held as Collateral	703,348		518,775
Mitsubishi UFJ Securities (USA), Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	663,555		625,703
Percent of Total Amount Outstanding	4.50%		4.00%
Company MBS Held as Collateral	698,836		656,046
Royal Bank of Canada
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	645,074		710,705
Percent of Total Amount Outstanding	4.40%		4.60%
Company MBS Held as Collateral	778,212		850,870
Morgan Stanley & Co. Incorporated
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	644,052		691,599
Percent of Total Amount Outstanding	4.40%		4.50%
Company MBS Held as Collateral	698,930		758,761
Scotia Capital
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	606,928		443,534
Percent of Total Amount Outstanding	4.10%		2.90%
Company MBS Held as Collateral	635,314		461,066
Rbs Securities Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	585,658		720,457
Percent of Total Amount Outstanding	4.00%		4.70%
Company MBS Held as Collateral	712,227		854,978
BNP Paribas Securities Corp.
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	533,500		471,372
Percent of Total Amount Outstanding	3.60%		3.10%
Company MBS Held as Collateral	595,846		499,106
Nomura Securities International, Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	449,541		578,265
Percent of Total Amount Outstanding	3.10%		3.70%
Company MBS Held as Collateral	475,390		608,193
Deutsche Bank Securities Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	413,703		423,405
Percent of Total Amount Outstanding	2.80%		2.70%
Company MBS Held as Collateral	467,375		468,939
KGS Alpha Capital Markets L P
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	308,908		202,677
Percent of Total Amount Outstanding	2.10%		1.30%
Company MBS Held as Collateral	326,124		214,033
Goldman, Sachs & Co
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	229,246		404,094
Percent of Total Amount Outstanding	1.60%		2.60%
Company MBS Held as Collateral	244,063		423,598
Guggenheim Liquidity Services, LLC
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	130,533		33,113
Percent of Total Amount Outstanding	0.90%		0.20%
Company MBS Held as Collateral	138,522		34,664
Barclays Capital Inc.
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	116,827		156,904
Percent of Total Amount Outstanding	0.80%		1.00%
Company MBS Held as Collateral	138,148		165,605
Cantor Fitzgerald & Co
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	111,921		68,261
Percent of Total Amount Outstanding	0.80%		0.40%
Company MBS Held as Collateral	119,656		71,910
Daiwa Capital Markets America Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	105,476		112,309
Percent of Total Amount Outstanding	0.70%		0.70%
Company MBS Held as Collateral	111,207		117,551
Td Securities
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	68,146		155,099
Percent of Total Amount Outstanding	0.50%		1.00%
Company MBS Held as Collateral	72,467		163,512
Mizuho Securities Usa Inc
Repurchase Agreement Counterparty [Line Items]
Amount Outstanding	58,591		53,962
Percent of Total Amount Outstanding	0.40%		0.30%
Company MBS Held as Collateral	70,720		62,423
Consolidation, Eliminations | Credit Suisse Securities (USA) LLC
Repurchase Agreement Counterparty [Line Items]
Company MBS Held as Collateral	$ 268,831		$ 133,837

[1] Includes $268.8 million of MBS held as collateral which are eliminated in consolidation.

[2] Includes $133.8 million of MBS held as collateral which are eliminated in consolidation.