Repurchase Agreements - Narrative (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Repurchase Agreements:
Repurchase agreements - outstanding	$ 101,033,146,000	$ 84,097,885,000	[1]
Repurchase agreements - weighted average borrowing rates	1.52%	1.59%
Repurchase agreements - weighted average remaining maturities (in days)	213 days	103 days
Repurchase agreements - collateral held, fair value	108,900,000,000	91,300,000,000
Repurchase agreements - accrued interest	$ 358,000,000	$ 337,000,000
Repurchase agreements - amount at risk not greater than percent	10.00%	10.00%

[1] Derived from the audited consolidated financial statements at December 31, 2011.