Repurchase Agreements - Narrative (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Repurchase Agreements:
Repurchase agreements - outstanding	$ 102,785,697	$ 84,097,885
Repurchase agreements - weighted average borrowing rates	1.53%	1.59%
Repurchase agreements - weighted average remaining maturities (in days)	191 days	103 days
Repurchase agreements - collateral held, fair value	109,200	91,300
Repurchase agreements - accrued interest	$ 363,800	$ 337,000