Carrying Values and Fair Values of All Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2012	Dec. 30, 2011
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	$ 25,256,043		$ 23,919,251
Cash and cash equivalents	88,987		168,424		288,691	347,045
Restricted cash	149,210		281,021
Unsettled purchased mortgage backed securities	133,597		138,338
Receivable for securities sold	7,251		1,587,535
Accrued interest receivable	73,957		77,113
Principal payments receivable	199,553		190,832
Debt security	15,000		15,000
Interest rate hedge asset	25,921
Forward purchase commitments			5,452
Futures contracts	5,587
Repurchase agreements	23,077,252		22,866,429
Payable for unsettled securities	136,373		137,121
Accrued interest payable	4,773		7,592
Forward purchase commitments	43,724
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	25,256,043		23,919,251
Cash and cash equivalents	88,987		168,424
Restricted cash	149,210		281,021
Unsettled purchased mortgage backed securities	133,597		138,338
Receivable for securities sold	7,251		1,587,535
Accrued interest receivable	73,957		77,113
Principal payments receivable	199,553		190,832
Debt security	15,000		15,000
Interest rate hedge asset	25,921
Short term investment	19,916	[1]	20,187	[1]
Forward purchase commitments			5,452	[1]
Futures contracts	5,587	[1]
Repurchase agreements	23,077,252		22,866,429
Payable for unsettled securities	136,373		137,121
Accrued interest payable	4,773		7,592
Interest rate hedge liability	157,869		243,945
Forward purchase commitments	43,724	[2]
Estimate of Fair Value, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	25,256,043		23,919,251
Cash and cash equivalents	88,987		168,424
Restricted cash	149,210		281,021
Unsettled purchased mortgage backed securities	133,597		138,338
Receivable for securities sold	7,251		1,587,535
Accrued interest receivable	73,957		77,113
Principal payments receivable	199,553		190,832
Debt security	14,216		13,981
Interest rate hedge asset	25,921
Short term investment	19,916	[1]	20,187	[1]
Forward purchase commitments			5,452	[1]
Futures contracts	5,587	[1]
Repurchase agreements	23,077,252		22,866,429
Payable for unsettled securities	136,373		137,121
Accrued interest payable	4,773		7,592
Interest rate hedge liability	157,869		243,945
Forward purchase commitments	$ 43,724	[2]

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

[2] This line included in accounts payable and other liabilities on the balance sheets.