Repurchase Agreements - Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Repurchase Agreements:
Repurchase agreements	$ 101,033,146	$ 84,097,885	[1]
Weighted average rate	0.61%	0.53%
1 day [Member]
Repurchase Agreements:
Repurchase agreements	3,585,234	508,647
Weighted average rate	0.28%	0.50%
2 to 29 days [Member]
Repurchase Agreements:
Repurchase agreements	27,234,957	33,780,070
Weighted average rate	0.45%	0.37%
30 to 59 days [Member]
Repurchase Agreements:
Repurchase agreements	18,726,669	28,346,380
Weighted average rate	0.43%	0.37%
60 to 89 days [Member]
Repurchase Agreements:
Repurchase agreements	4,795,439	3,699,425
Weighted average rate	0.41%	0.93%
90 to 119 days [Member]
Repurchase Agreements:
Repurchase agreements	11,309,846	6,781,137
Weighted average rate	0.49%	0.37%
Over 120 days [Member]
Repurchase Agreements:
Repurchase agreements	$ 35,381,001	$ 10,982,226
Weighted average rate	0.93%	1.39%

[1] Derived from the audited consolidated financial statements at December 31, 2011.