Carrying Values and Fair Values of All Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	$ 23,919,251		$ 17,741,873
Cash and cash equivalents	168,424		347,045		112,626	225,828
Restricted cash	281,021		237,014
Unsettled purchased mortgage backed securities	138,338		49,630
Receivable for securities sold	1,587,535
Accrued interest receivable	77,113		63,025
Principal payments receivable	190,832		105,333
Debt Security	15,000		15,000
Forward purchase commitments	5,452		6,327
Repurchase agreements	22,866,429		16,162,375
Payable for unsettled securities	137,121		48,999
Accrued interest payable	7,592		4,596
Interest rate hedge liability	243,945		219,167
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	23,919,251		17,741,873
Cash and cash equivalents	168,424		347,045
Restricted cash	281,021		237,014
Unsettled purchased mortgage backed securities	138,338		49,630
Receivable for securities sold	1,587,535
Accrued interest receivable	77,113		63,025
Principal payments receivable	190,832		105,333
Debt Security	15,000		15,000
Short term investment	20,187	[1]	20,221	[1]
Forward purchase commitments	5,452	[1]	6,327	[1]
Repurchase agreements	22,866,429		16,162,375
Payable for unsettled securities	137,121		48,999
Accrued interest payable	7,592		4,596
Interest rate hedge liability	243,945		219,167
Estimate of Fair Value, Fair Value Disclosure
Fair Value of Financial Instruments [Line Items]
Mortgage-backed securities	23,919,251		17,741,873
Cash and cash equivalents	168,424		347,045
Restricted cash	281,021		237,014
Unsettled purchased mortgage backed securities	138,338		49,630
Receivable for securities sold	1,587,535
Accrued interest receivable	77,113		63,025
Principal payments receivable	190,832		105,333
Debt Security	13,981		13,811
Short term investment	20,187	[1]	20,221	[1]
Forward purchase commitments	5,452	[1]	6,327	[1]
Repurchase agreements	22,866,429		16,162,375
Payable for unsettled securities	137,121		48,999
Accrued interest payable	7,592		4,596
Interest rate hedge liability	$ 243,945		$ 219,167

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