Activity of Credit Component of Other-than-Temporary Impairments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Balance at beginning of period	$ 45,611	$ 73,698	$ 50,852	$ 78,126
Initial credit impairments		55		161
Subsequent credit impairments		91		91
Securities sold, or expected to sell	(2,191)		(2,191)
Securities with no outstanding principal at period end	(746)	(6,790)	(5,987)	(11,324)
Balance at End of Period	$ 42,674	$ 67,054	$ 42,674	$ 67,054