Borrowings - Repurchase Agreements (Detail) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) counterparties		Dec. 31, 2014 USD ($) counterparties
Repurchase Agreement Counterparty
Amount outstanding		$ 12,126,048		$ 13,622,677
Percent of total amount outstanding		100.00%		100.00%
Securities sold under agreements to repurchase fair value of collateral		$ 13,460,288		$ 15,260,264
Number of counterparties | counterparties		9		10
Credit Suisse Securities (USA) LLC
Repurchase Agreement Counterparty
Amount outstanding				$ 1,517,530
Percent of total amount outstanding				11.10%
Securities sold under agreements to repurchase fair value of collateral	[1]			$ 1,925,973
HSBC Securities (USA) Inc.
Repurchase Agreement Counterparty
Amount outstanding		$ 1,566,747		$ 1,190,769
Percent of total amount outstanding		12.90%		8.70%
Securities sold under agreements to repurchase fair value of collateral		$ 1,611,020		$ 1,225,194
Royal Bank of Canada
Repurchase Agreement Counterparty
Amount outstanding		$ 1,148,480		$ 1,057,798
Percent of total amount outstanding		9.50%		7.80%
Securities sold under agreements to repurchase fair value of collateral		$ 1,383,839		$ 1,278,612
ING Financial Market LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 1,050,548		$ 767,733
Percent of total amount outstanding		8.70%		5.60%
Securities sold under agreements to repurchase fair value of collateral		$ 1,112,102		$ 820,166
South Street Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 799,783		$ 961,938
Percent of total amount outstanding		6.60%		7.10%
Securities sold under agreements to repurchase fair value of collateral		$ 838,600		$ 1,020,054
Pierpont Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 786,623		$ 601,222
Percent of total amount outstanding		6.50%		4.40%
Securities sold under agreements to repurchase fair value of collateral		$ 814,804		$ 627,534
Industrial And Commercial Bank Of China Financial Services LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 695,933		$ 682,193
Percent of total amount outstanding		5.70%		5.00%
Securities sold under agreements to repurchase fair value of collateral		$ 730,941		$ 716,989
Mitsubishi UFJ Securities (USA), Inc.
Repurchase Agreement Counterparty
Amount outstanding		$ 627,383		$ 710,058
Percent of total amount outstanding		5.20%		5.20%
Securities sold under agreements to repurchase fair value of collateral		$ 657,201		$ 744,836
JP Morgan Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 622,665		$ 698,856
Percent of total amount outstanding		5.10%		5.10%
Securities sold under agreements to repurchase fair value of collateral		$ 728,502		$ 814,896
Citigroup Global Markets Inc.
Repurchase Agreement Counterparty
Amount outstanding		$ 585,632		$ 979,247
Percent of total amount outstanding		4.80%		7.20%
Securities sold under agreements to repurchase fair value of collateral		$ 725,882		$ 1,157,265	[2]
Scotia Capital
Repurchase Agreement Counterparty
Amount outstanding		$ 576,137		$ 521,778
Percent of total amount outstanding		4.80%		3.80%
Securities sold under agreements to repurchase fair value of collateral		$ 598,343		$ 542,044
BNP Paribas Securities Corp.
Repurchase Agreement Counterparty
Amount outstanding		$ 474,053		$ 559,658
Percent of total amount outstanding		3.90%		4.10%
Securities sold under agreements to repurchase fair value of collateral		$ 530,584		$ 622,749
Wells Fargo Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 463,673		$ 627,071
Percent of total amount outstanding		3.80%		4.60%
Securities sold under agreements to repurchase fair value of collateral		$ 551,667		$ 754,706
Goldman Sachs & Co.
Repurchase Agreement Counterparty
Amount outstanding		$ 428,799
Percent of total amount outstanding		3.50%
Securities sold under agreements to repurchase fair value of collateral		$ 552,549
KGS Alpha Capital Markets L.P.
Repurchase Agreement Counterparty
Amount outstanding		$ 380,286		$ 407,920
Percent of total amount outstanding		3.10%		3.00%
Securities sold under agreements to repurchase fair value of collateral		$ 400,758		$ 430,241
Banc Of America Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 380,520		$ 791,196
Percent of total amount outstanding		3.10%		5.90%
Securities sold under agreements to repurchase fair value of collateral		$ 442,801		$ 875,984	[3]
Morgan Stanley & Co. Incorporated
Repurchase Agreement Counterparty
Amount outstanding		$ 273,124		$ 589,950
Percent of total amount outstanding		2.30%		4.30%
Securities sold under agreements to repurchase fair value of collateral		$ 320,484		$ 632,002
Guggenheim Liquidity Services, LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 265,709
Percent of total amount outstanding		2.20%
Securities sold under agreements to repurchase fair value of collateral		$ 279,345
All Other Counterparties
Repurchase Agreement Counterparty
Amount outstanding		$ 999,953	[4]	$ 957,760	[5]
Percent of total amount outstanding		8.30%	[4]	7.10%	[5]
Securities sold under agreements to repurchase fair value of collateral		$ 1,180,866	[4]	$ 1,071,019	[5]
Consolidation, Eliminations
Repurchase Agreement Counterparty
Securities sold under agreements to repurchase fair value of collateral				403,200
Consolidation, Eliminations | Credit Suisse Securities (USA) LLC
Repurchase Agreement Counterparty
Securities sold under agreements to repurchase fair value of collateral				276,100
Consolidation, Eliminations | Citigroup Global Markets Inc.
Repurchase Agreement Counterparty
Securities sold under agreements to repurchase fair value of collateral				20,300
Consolidation, Eliminations | Banc Of America Securities LLC
Repurchase Agreement Counterparty
Securities sold under agreements to repurchase fair value of collateral				$ 106,800

[1] Includes $276.1 million of MBS held as collateral which are eliminated in consolidation.

[2] Includes $20.3 million of MBS held as collateral which are eliminated in consolidation.

[3] Includes $106.8 million of MBS held as collateral which are eliminated in consolidation.

[4] Represents amounts outstanding with nine counterparties.

[5] Represents amounts outstanding with ten counterparties.