Carrying Values and Fair Values of All Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011	Dec. 31, 2010
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	$ 22,367,121		$ 17,741,873
Unsettled purchased mortgage-backed securities	829,060		49,630
Cash and cash equivalents	288,691		347,045		233,213	112,626
Restricted cash	278,178		237,014
Accrued interest receivable	73,856		63,025
Principal payments receivable	138,712		105,333
Debt Security	15,000		15,000
Forward purchase commitments	6,021				6,327
Repurchase agreements	20,152,860		16,162,375
Payable for unsettled securities	827,732		48,999
Accrued interest payable	4,393		4,596
Interest rate hedge liability	250,933				219,167
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	22,367,121		17,741,873
Unsettled purchased mortgage-backed securities	829,060		49,630
Cash and cash equivalents	288,691		347,045
Restricted cash	278,178		237,014
Accrued interest receivable	73,856		63,025
Principal payments receivable	138,712		105,333
Debt Security	15,000		15,000
Short term investment	20,018	[1]	20,221	[1]
Forward purchase commitments	6,021	[1]	6,327	[1]
Repurchase agreements	20,152,860		16,162,375
Payable for unsettled securities	827,732		48,999
Accrued interest payable	4,393		4,596
Interest rate hedge liability	250,933		219,167
Estimate of Fair Value, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	22,367,121		17,741,873
Unsettled purchased mortgage-backed securities	829,060		49,630
Cash and cash equivalents	288,691		347,045
Restricted cash	278,178		237,014
Accrued interest receivable	73,856		63,025
Principal payments receivable	138,712		105,333
Debt Security	13,879		13,811
Short term investment	20,018	[1]	20,221	[1]
Forward purchase commitments	6,021	[1]	6,327	[1]
Repurchase agreements	20,152,860		16,162,375
Payable for unsettled securities	827,732		48,999
Accrued interest payable	4,393		4,596
Interest rate hedge liability	$ 250,933		$ 219,167

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