Repurchase Agreements - Remaining Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Repurchase Agreements:
Repurchase agreements	$ 102,785,697	$ 84,097,885
Weighted average rate	0.63%	0.53%
1 day [Member]
Repurchase Agreements:
Repurchase agreements		508,647
Weighted average rate		0.50%
2 to 29 days [Member]
Repurchase Agreements:
Repurchase agreements	33,191,448	33,780,070
Weighted average rate	0.50%	0.37%
30 to 59 days [Member]
Repurchase Agreements:
Repurchase agreements	28,383,851	28,346,380
Weighted average rate	0.45%	0.37%
60 to 89 days [Member]
Repurchase Agreements:
Repurchase agreements	8,602,680	3,699,425
Weighted average rate	0.42%	0.93%
90 to 119 days [Member]
Repurchase Agreements:
Repurchase agreements	4,804,671	6,781,137
Weighted average rate	0.57%	0.37%
Over 120 days [Member]
Repurchase Agreements:
Repurchase agreements	$ 27,803,047	$ 10,982,226
Weighted average rate	1.03%	1.39%