Repurchase Agreements - Remaining Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Repurchase Agreements:
Repurchase agreements	$ 61,781,001	$ 102,785,697
Weighted average rate	0.68%	0.63%
1 day
Repurchase Agreements:
Repurchase agreements
Weighted average rate	0.00%	0.00%
2 to 29 days
Repurchase Agreements:
Repurchase agreements	21,171,574	33,191,448
Weighted average rate	0.36%	0.50%
30 to 59 days
Repurchase Agreements:
Repurchase agreements	13,373,921	28,383,851
Weighted average rate	0.43%	0.45%
60 to 89 days
Repurchase Agreements:
Repurchase agreements	3,592,266	8,602,680
Weighted average rate	0.44%	0.42%
90 to 119 days
Repurchase Agreements:
Repurchase agreements	4,010,334	4,804,671
Weighted average rate	0.52%	0.57%
Over 120 days
Repurchase Agreements:
Repurchase agreements	$ 19,632,906	$ 27,803,047
Weighted average rate	1.29%	1.03%