Repurchase Agreements - Remaining Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Repurchase Agreements:
Repurchase agreements	$ 60,477,378		$ 71,361,926	[1]
Weighted average rate	0.70%		0.61%
1 day
Repurchase Agreements:
Repurchase agreements	7,658,440
Weighted average rate	0.64%		0.00%
2 to 29 days
Repurchase Agreements:
Repurchase agreements	16,080,033		28,354,167
Weighted average rate	0.39%		0.35%
30 to 59 days
Repurchase Agreements:
Repurchase agreements	7,326,177		17,336,469
Weighted average rate	0.40%		0.43%
60 to 89 days
Repurchase Agreements:
Repurchase agreements	9,534,614		4,040,677
Weighted average rate	0.40%		0.38%
90 to 119 days
Repurchase Agreements:
Repurchase agreements	4,677,222		2,945,495
Weighted average rate	0.50%		0.50%
Over 120 days
Repurchase Agreements:
Repurchase agreements	$ 15,200,892	[2]	$ 18,685,118	[2]
Weighted average rate	1.45%	[2]	1.24%	[2]

[1] Derived from the audited consolidated financial statements at December 31, 2014.

[2] Approximately 18% and 16% of the total repurchase agreements had a remaining maturity over 1 year as of March 31, 2015 and December 31, 2014, respectively.