Aggregate Fair Value and Notional Amount of Derivative Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Fair Value	$ 13,770	$ (48,109)
Notional Amount	2,537,000	1,453,000
Derivative Assets
Derivative [Line Items]
Fair Value	43,341	2,972
Notional Amount	2,034,000	722,000
Derivative Assets | Interest Rate Swaps
Derivative [Line Items]
Fair Value	32,742	739
Notional Amount	1,044,000	147,000
Derivative Assets | TBAs
Derivative [Line Items]
Fair Value	8,655
Notional Amount	345,000
Derivative Assets | Futures
Derivative [Line Items]
Fair Value	328
Notional Amount	30,000
Derivative Assets | Swaptions
Derivative [Line Items]
Fair Value	1,616	2,233
Notional Amount	615,000	575,000
Derivative Liabilities
Derivative [Line Items]
Fair Value	(29,571)	(51,081)
Notional Amount	503,000	731,000
Derivative Liabilities | Interest Rate Swaps
Derivative [Line Items]
Fair Value	(219)	(1,893)
Notional Amount	20,500	357,500
Derivative Liabilities | TBAs
Derivative [Line Items]
Fair Value	(1,139)
Notional Amount	105,000
Derivative Liabilities | Futures
Derivative [Line Items]
Fair Value	(647)	(607)
Notional Amount	238,000	234,000
Derivative Liabilities | Cash Flow Hedging | Interest Rate Swaps
Derivative [Line Items]
Fair Value	(27,566)	(48,581)
Notional Amount	$ 139,500	$ 139,500