Summary of Gross Amounts, Amounts Offset and net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013		Jan. 31, 2014
Repurchase Agreements:
Gross amounts -reverse repurchase agreements	$ 1,766,563	$ 2,524,980
Amounts offset - reverse repurchase agreement	(1,322,188)	(2,424,980)
Netted amounts -reverse repurchase	444,375	100,000	[1]
Gross amounts -repurchase agreement	65,866,137	64,205,981
Amounts offset -repurchase agreement	(1,322,188)	(2,424,980)
Netted amounts -repurchase agreement	$ 64,543,949	$ 61,781,001	[1]	$ 399,500

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