Repurchase Agreements - Remaining Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Repurchase Agreements:
Repurchase agreements	$ 100,322,942	$ 102,785,697	[1]
Weighted average rate	0.59%	0.63%
1 day
Repurchase Agreements:
Repurchase agreements	6,664,087
Weighted average rate	0.32%
2 to 29 days
Repurchase Agreements:
Repurchase agreements	26,451,593	33,191,448
Weighted average rate	0.46%	0.50%
30 to 59 days
Repurchase Agreements:
Repurchase agreements	13,129,666	28,383,851
Weighted average rate	0.44%	0.45%
60 to 89 days
Repurchase Agreements:
Repurchase agreements	8,705,572	8,602,680
Weighted average rate	0.36%	0.42%
90 to 119 days
Repurchase Agreements:
Repurchase agreements	11,103,023	4,804,671
Weighted average rate	0.47%	0.57%
Over 120 days
Repurchase Agreements:
Repurchase agreements	$ 34,269,001	$ 27,803,047
Weighted average rate	0.91%	1.03%

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