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, 2014	Dec. 31, 2013
Repurchase Agreements:
Gross amounts -reverse repurchase agreements	$ 700,000	$ 2,524,980
Amounts offset - reverse repurchase agreement	(600,000)	(2,424,980)
Netted amounts -reverse repurchase	100,000	100,000
Gross amounts -repurchase agreement	71,961,926	64,205,981
Amounts offset -repurchase agreement	(600,000)	(2,424,980)
Netted amounts -repurchase agreement	$ 71,361,926	$ 61,781,001