Borrowings - Repurchase Agreements (Detail) $ in Thousands		6 Months Ended
		Jun. 30, 2015 USD ($) counterparties		Jun. 30, 2014 counterparties	Dec. 31, 2014 USD ($)
Repurchase Agreement Counterparty
Amount outstanding		$ 13,174,860			$ 13,622,677
Percent of total amount outstanding		100.00%			100.00%
Company MBS and GSE CRT held as collateral		$ 14,683,456			$ 15,260,264
Number of counterparties | counterparties		8		10
HSBC Securities (USA) Inc
Repurchase Agreement Counterparty
Amount outstanding		$ 1,202,021			$ 1,190,769
Percent of total amount outstanding		9.20%			8.70%
Company MBS and GSE CRT held as collateral		$ 1,239,389			$ 1,225,194
Citigroup Global Markets Inc
Repurchase Agreement Counterparty
Amount outstanding		$ 1,096,155			$ 979,247
Percent of total amount outstanding		8.30%			7.20%
Company MBS and GSE CRT held as collateral		$ 1,289,766	[1]		$ 1,157,265	[2]
Royal Bank of Canada
Repurchase Agreement Counterparty
Amount outstanding		$ 1,071,275			$ 1,057,798
Percent of total amount outstanding		8.10%			7.80%
Company MBS and GSE CRT held as collateral		$ 1,293,195			$ 1,278,612
South Street Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 882,368			$ 961,938
Percent of total amount outstanding		6.70%			7.10%
Company MBS and GSE CRT held as collateral		$ 928,294			$ 1,020,054
CRT Capital Group LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 753,422
Percent of total amount outstanding		5.70%
Company MBS and GSE CRT held as collateral		$ 791,630
Goldman, Sachs & Co
Repurchase Agreement Counterparty
Amount outstanding		$ 676,719
Percent of total amount outstanding		5.10%
Company MBS and GSE CRT held as collateral		$ 816,439
Industrial And Commercial Bank Of China Financial Services LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 668,548			$ 682,193
Percent of total amount outstanding		5.10%			5.00%
Company MBS and GSE CRT held as collateral		$ 703,486			$ 716,989
Mitsubishi UFJ Securities (USA), Inc
Repurchase Agreement Counterparty
Amount outstanding		$ 666,581			$ 710,058
Percent of total amount outstanding		5.10%			5.20%
Company MBS and GSE CRT held as collateral		$ 700,639			$ 744,836
JP Morgan Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 662,483			$ 698,856
Percent of total amount outstanding		5.00%			5.10%
Company MBS and GSE CRT held as collateral		$ 762,158			$ 814,896
Banc of America Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 655,434			$ 791,196
Percent of total amount outstanding		5.00%			5.90%
Company MBS and GSE CRT held as collateral		$ 750,231	[3]		$ 875,984	[4]
Pierpont Securities LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 611,156			$ 601,222
Percent of total amount outstanding		4.60%			4.40%
Company MBS and GSE CRT held as collateral		$ 638,543			$ 627,534
Wells Fargo Securities, LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 602,134			$ 627,071
Percent of total amount outstanding		4.60%			4.60%
Company MBS and GSE CRT held as collateral		$ 725,679			$ 754,706
Scotia Capital
Repurchase Agreement Counterparty
Amount outstanding		$ 544,117			$ 521,778
Percent of total amount outstanding		4.10%			3.80%
Company MBS and GSE CRT held as collateral		$ 565,837			$ 542,044
BNP Paribas Securities Corp
Repurchase Agreement Counterparty
Amount outstanding		$ 518,792			$ 559,658
Percent of total amount outstanding		3.90%			4.10%
Company MBS and GSE CRT held as collateral		$ 577,427			$ 622,749
ING Financial Market LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 490,120			$ 767,733
Percent of total amount outstanding		3.70%			5.60%
Company MBS and GSE CRT held as collateral		$ 521,261			$ 820,166
Morgan Stanley & Co. Incorporated
Repurchase Agreement Counterparty
Amount outstanding		$ 474,048			$ 589,950
Percent of total amount outstanding		3.60%			4.30%
Company MBS and GSE CRT held as collateral		$ 519,377			$ 632,002
Credit Suisse Securities (USA) LLC
Repurchase Agreement Counterparty
Amount outstanding		$ 468,477			$ 1,517,530
Percent of total amount outstanding		3.60%			11.10%
Company MBS and GSE CRT held as collateral		$ 611,182	[5]		$ 1,925,973	[6]
KGS Alpha Capital Markets LP
Repurchase Agreement Counterparty
Amount outstanding		$ 406,091			$ 407,920
Percent of total amount outstanding		3.10%			3.00%
Company MBS and GSE CRT held as collateral		$ 427,033			$ 430,241
All Other Counterparties
Repurchase Agreement Counterparty
Amount outstanding		$ 724,919	[7]		$ 957,760	[8]
Percent of total amount outstanding		5.50%	[7]		7.10%	[8]
Company MBS and GSE CRT held as collateral		$ 821,890	[7]		$ 1,071,019	[8]
Consolidation, Eliminations
Repurchase Agreement Counterparty
Company MBS and GSE CRT held as collateral		419,400			403,200
Consolidation, Eliminations | Citigroup Global Markets Inc
Repurchase Agreement Counterparty
Company MBS and GSE CRT held as collateral	[9],[10],[11]	124,300			20,300
Consolidation, Eliminations | Banc of America Securities LLC
Repurchase Agreement Counterparty
Company MBS and GSE CRT held as collateral	[9],[10],[11]	85,400			106,800
Consolidation, Eliminations | Credit Suisse Securities (USA) LLC
Repurchase Agreement Counterparty
Company MBS and GSE CRT held as collateral	[9],[10],[11]	$ 209,700			$ 276,100

[1] Includes $209.7 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 $124.3 million of MBS held as collateral which are eliminated in consolidation.

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

[5] Includes $85.4 million of MBS held as collateral which are eliminated in consolidation.

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

[7] Represents amounts outstanding with eight counterparties.

[8] Represents amounts outstanding with ten counterparties.

[9] Amounts represent collateral pledged that is available to be offset against liability balances associated with repurchase agreements, secured loans and derivatives.

[10] Cash collateral received on the Company's derivatives was $6.5 million and $14.9 million at June 30, 2015 and December 31, 2014, respectively. Non-cash collateral received on the Company's derivatives was $12.3 million and $10.8 million at June 30, 2015 and December 31, 2014. Cash collateral posted by the Company on its derivatives was $65.1 million and $57.6 million at June 30, 2015 and December 31, 2014, respectively.

[11] The fair value of securities pledged against IAS Services LLC's borrowing under secured loans was $1.8 billion and $1.5 billion at June 30, 2015 and December 31, 2014, respectively.