Summary of Gross Amounts, Amounts Offset and net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Detail) (USD $) In Thousands, unless otherwise specified	1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2013	Dec. 31, 2012
Repurchase Agreements:
Gross amounts -reverse repurchase agreements		$ 7,041,379	$ 3,650,053
Amounts offset - reverse repurchase agreement		(2,107,914)	(1,838,958)
Netted amounts -reverse repurchase	4,933,465	4,933,465	1,811,095	[1]
Gross amounts -repurchase agreement	102,430,856		104,624,655
Amounts offset -repurchase agreement	(2,107,914)		(1,838,958)
Netted amounts -repurchase agreement	$ 100,322,942	$ 100,322,942	$ 102,785,697	[1]

[1] Derived from the audited consolidated financial statements at December 31, 2012.