Summary of Gross Amounts, Amounts Offset and net Amounts of Repurchase Agreement and Reverse Repurchase Agreement (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Repurchase Agreements:
Gross amounts -reverse repurchase agreements	$ 2,524,980	$ 3,650,053
Amounts offset - reverse repurchase agreement	(2,424,980)	(1,838,958)
Netted amounts -reverse repurchase	100,000	1,811,095
Gross amounts -repurchase agreement	64,205,981	104,624,655
Amounts offset -repurchase agreement	(2,424,980)	(1,838,958)
Netted amounts -repurchase agreement	$ 61,781,001	$ 102,785,697