Repurchase Agreements - Narrative (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Repurchase Agreements:
Repurchase agreements - outstanding	$ 69,211,309,000	$ 102,785,697,000	[1]
Repurchase agreements - weighted average borrowing rates	2.02%	1.53%
Repurchase agreements - weighted average remaining maturities (in days)	200 days	191 days
Repurchase agreements - collateral held, fair value	75,900,000,000	109,200,000,000
Repurchase agreements - accrued interest	$ 252,000,000	$ 363,800,000

[1] Derived from the audited consolidated financial statements at December 31, 2012.