Repurchase Agreements - Remaining Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Jan. 31, 2014	Dec. 31, 2013
Repurchase Agreements:
Repurchase agreements	$ 64,543,949	$ 399,500	$ 61,781,001	[1]
Weighted average rate	0.65%		0.68%
1 day
Repurchase Agreements:
Repurchase agreements	6,065,530
Weighted average rate	0.29%		0.00%
2 to 29 days
Repurchase Agreements:
Repurchase agreements	12,308,123		21,171,574
Weighted average rate	0.34%		0.36%
30 to 59 days
Repurchase Agreements:
Repurchase agreements	17,122,926		13,373,921
Weighted average rate	0.43%		0.43%
60 to 89 days
Repurchase Agreements:
Repurchase agreements	3,842,791		3,592,266
Weighted average rate	0.42%		0.44%
90 to 119 days
Repurchase Agreements:
Repurchase agreements	6,941,938		4,010,334
Weighted average rate	0.55%		0.52%
Over 120 days
Repurchase Agreements:
Repurchase agreements	$ 18,262,641		$ 19,632,906
Weighted average rate	1.26%		1.29%

[1] Derived from the audited consolidated financial statements at December 31, 2013.