Repurchase Agreements - Remaining Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Repurchase Agreements:
Repurchase agreements	$ 71,361,926		$ 61,781,001
Weighted average rate	0.61%		0.68%
1 day
Repurchase Agreements:
Weighted average rate	0.00%		0.00%
2 to 29 days
Repurchase Agreements:
Repurchase agreements	28,354,167		21,171,574
Weighted average rate	0.35%		0.36%
30 to 59 days
Repurchase Agreements:
Repurchase agreements	17,336,469		13,373,921
Weighted average rate	0.43%		0.43%
60 to 89 days
Repurchase Agreements:
Repurchase agreements	4,040,677		3,592,266
Weighted average rate	0.38%		0.44%
90 to 119 days
Repurchase Agreements:
Repurchase agreements	2,945,495		4,010,334
Weighted average rate	0.50%		0.52%
Over 120 days
Repurchase Agreements:
Repurchase agreements	$ 18,685,118	[1]	$ 19,632,906	[1]
Weighted average rate	1.24%	[1]	1.29%	[1]

[1] Approximately 15% and 16% of the total repurchase agreements had a remaining maturity over 1 year as of December 31, 2014 and 2013, respectively.