Repurchase Agreements - Narrative (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Repurchase Agreements:
Repurchase agreements - outstanding	$ 100,322,942,000	$ 102,785,697,000	[1]
Repurchase agreements - weighted average borrowing rates	1.49%	1.53%
Repurchase agreements - weighted average remaining maturities (in days)	198 days	191 days
Repurchase agreements - collateral held, fair value	103,100,000,000	109,200,000,000
Repurchase agreements - accrued interest	$ 328,200,000	$ 363,800,000

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