Repurchase Agreements - Narrative (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Short-term Debt [Line Items]
Repurchase agreements outstanding	$ 1,658,906,000	$ 2,672,989,000	[1]
Weighted average borrowing rates	0.48%	0.35%
Weighted average remaining maturities, in days	64 days	48 days
RMBS pledged as collateral under repurchase agreements, at estimated fair value	1,700,000,000	2,800,000,000
The average daily balances of the Company's repurchase agreements for the quarters	$ 2,300,000,000	$ 3,800,000,000
Description of variable interest rate	The interest rates of these repurchase agreements are generally indexed to the one-month or the three-month LIBOR rate and re-price accordingly.

[1] Derived from the audited consolidated financial statements.