Repurchase Agreements - Narrative (Detail) (USD $)	Jun. 30, 2011	Dec. 31, 2010
Disclosure of Repurchase Agreements [Line Items]
Repurchase agreements - outstanding	$ 78,447,165,000	$ 65,533,537,000	[1]
Repurchase agreements - weighted average borrowing rates	1.68%	1.84%
Repurchase agreements - weighted average remaining maturities (in days)	130	127
Repurchase agreements - collateral held, fair value	83,100,000,000	69,500,000,000
Repurchase agreements - amount at risk not greater than percent	10.00%	10.00%
Repurchase agreements with right to call prior to maturity, fair value	263,800,000	313,200,000
Over 1 year [Member]
Disclosure of Repurchase Agreements [Line Items]
Repurchase agreements - outstanding	1,000,000,000	500,000,000
Repurchase agreements with a term of over one year - fair value	1,000,000,000	513,300,000
Repurchase agreements with right to call prior to maturity, amount [Member]
Disclosure of Repurchase Agreements [Line Items]
Repurchase agreements - outstanding	$ 5,200,000,000	$ 5,900,000,000

[1] Derived from the audited consolidated financial statements at December 31, 2010.