Repurchase Agreements - Remaining Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Repurchase Agreements:
Repurchase agreements	$ 69,610,722		$ 61,781,001	[1]
Weighted average rate	0.58%		0.68%
1 day
Repurchase Agreements:
Repurchase agreements	2,609,132
Weighted average rate	0.15%		0.00%
2 to 29 days
Repurchase Agreements:
Repurchase agreements	18,032,715		21,171,574
Weighted average rate	0.30%		0.36%
30 to 59 days
Repurchase Agreements:
Repurchase agreements	14,241,967		13,373,921
Weighted average rate	0.37%		0.43%
60 to 89 days
Repurchase Agreements:
Repurchase agreements	2,871,206		3,592,266
Weighted average rate	0.38%		0.44%
90 to 119 days
Repurchase Agreements:
Repurchase agreements	10,548,578		4,010,334
Weighted average rate	0.37%		0.52%
Over 120 days
Repurchase Agreements:
Repurchase agreements	$ 21,307,124	[2]	$ 19,632,906	[2]
Weighted average rate	1.14%	[2]	1.29%	[2]

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

[2] Approximately 12% and 16% of the total repurchase agreements had a remaining maturity over 1 year as of September 30, 2014 and December 31, 2013, respectively.