Activity of Credit Component of Other-than-Temporary Impairments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Balance at beginning of period	$ 42,674	$ 67,054	$ 50,852	$ 78,126
Initial credit impairments		164		325
Subsequent credit impairments		58		149
Securities sold, or expected to sell	(3,288)	(9,069)	(5,479)	(9,069)
Securities with no outstanding principal at period end	(764)	(4,310)	(6,751)	(15,634)
Balance at End of Period	$ 38,622	$ 53,897	$ 38,622	$ 53,897