Repurchase Agreements - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012 Person	Dec. 31, 2011 Person
Repurchase Agreement Counterparty [Line Items]
Number of counterparties in repurchase agreements	24,000	23,000
Weighted average contractual maturity of the repurchase agreements outstanding (in months)	24 days	24 days
Repurchase agreement amount	$ 20,152,860	$ 16,162,375
Weighted Average Contractual Rate	0.41%	0.37%
Fair market value of assets pledged as collateral under repurchase agreements	$ 21,231,528	$ 17,061,970