Carrying Values and Fair Values of All Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012	Dec. 31, 2011
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	$ 19,843,830		$ 23,919,251
Cash and cash equivalents	547,536		168,424		150,846	347,045
Restricted cash	241,853		281,021
Unsettled purchased mortgage backed securities	628,684		138,338
Receivable for securities sold	548,942		1,587,535
Accrued interest receivable	62,921		77,113
Principal payments receivable	156,758		190,832
Debt security	15,000		15,000
Interest rate hedge asset	13,626
Forward purchase commitments	6,904		5,452
Repurchase agreements	18,829,771		22,866,429
Payable for unsettled securities	625,758		137,121
Accrued interest payable	3,901		7,592
Interest rate hedge liability	147,954		243,945
Futures Contracts	44,593
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	19,843,830		23,919,251
Cash and cash equivalents	547,536		168,424
Restricted cash	241,853		281,021
Unsettled purchased mortgage backed securities	628,684		138,338
Receivable for securities sold	548,942		1,587,535
Accrued interest receivable	62,921		77,113
Principal payments receivable	156,758		190,832
Debt security	15,000		15,000
Interest rate hedge asset	13,626
Short term investment	20,044	[1]	20,187	[1]
Forward purchase commitments	6,904	[1]	5,452	[1]
Repurchase agreements	18,829,771		22,866,429
Payable for unsettled securities	625,758		137,121
Accrued interest payable	3,901		7,592
Interest rate hedge liability	147,954		243,945
Futures Contracts	44,593
Estimate of Fair Value, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	19,843,830		23,919,251
Cash and cash equivalents	547,536		168,424
Restricted cash	241,853		281,021
Unsettled purchased mortgage backed securities	628,684		138,338
Receivable for securities sold	548,942		1,587,535
Accrued interest receivable	62,921		77,113
Principal payments receivable	156,758		190,832
Debt security	14,279		13,981
Interest rate hedge asset	13,626
Short term investment	20,044	[1]	20,187	[1]
Forward purchase commitments	6,904	[1]	5,452	[1]
Repurchase agreements	18,829,771		22,866,429
Payable for unsettled securities	625,758		137,121
Accrued interest payable	3,901		7,592
Interest rate hedge liability	147,954		243,945
Futures Contracts	$ 44,593

[1] These lines are included in other assets on the consolidated balance sheets.