Carrying Values and Fair Values of All Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012		Mar. 31, 2012	Dec. 31, 2011
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	$ 25,162,730		$ 23,919,251
Cash and cash equivalents	315,253		168,424		715,745	347,045
Restricted cash	239,417		281,021
Unsettled purchased mortgage backed securities	113,171		138,338
Receivable for securities sold			1,587,535
Accrued interest receivable	74,691		77,113
Principal payments receivable	151,200		190,832
Debt security	15,000		15,000
Forward purchase commitments	6,476		5,452
Futures contracts	51
Repurchase agreements	22,586,932		22,866,429
Payable for unsettled securities	138,688		137,121
Accrued interest payable	5,040		7,592
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	25,162,730		23,919,251
Cash and cash equivalents	315,253		168,424
Restricted cash	239,417		281,021
Unsettled purchased mortgage backed securities	113,171		138,338
Receivable for securities sold			1,587,535
Accrued interest receivable	74,691		77,113
Principal payments receivable	151,200		190,832
Debt security	15,000		15,000
Short term investment	20,151	[1]	20,187	[1]
Forward purchase commitments	6,476	[1]	5,452	[1]
Futures contracts	51	[1]
Repurchase agreements	22,586,932		22,866,429
Payable for unsettled securities	138,688		137,121
Accrued interest payable	5,040		7,592
Interest rate hedge liability	220,040		243,945
Estimate of Fair Value, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	25,162,730		23,919,251
Cash and cash equivalents	315,253		168,424
Restricted cash	239,417		281,021
Unsettled purchased mortgage backed securities	113,171		138,338
Receivable for securities sold			1,587,535
Accrued interest receivable	74,691		77,113
Principal payments receivable	151,200		190,832
Debt security	14,014		13,981
Short term investment	20,151	[1]	20,187	[1]
Forward purchase commitments	6,476	[1]	5,452	[1]
Futures contracts	51	[1]
Repurchase agreements	22,586,932		22,866,429
Payable for unsettled securities	138,688		137,121
Accrued interest payable	5,040		7,592
Interest rate hedge liability	$ 220,040		$ 243,945

[1] These lines are included in Other assets on the balance sheets