Repurchase Agreements - Maturities (Detail) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements	$ 78,447,165	$ 65,533,537	[1]
1 day [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements	6,751,759
2 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements	29,838,769	32,669,341
30 to 59 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements	13,627,318	13,767,522
60 to 89 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements	5,269,428	4,776,597
90 to 119 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements	6,413,382	6,068,376
Over 120 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements	$ 16,546,509	$ 8,251,701

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