Changes in Credit Loss Component of Cumulative Other-Than-Temporary Impairment Losses on Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Balance, beginning of period	$ 29,251	$ 11,076	$ 29,070	$ 10,500
Additions: Subsequent credit impairments	4,697		5,247	825
Reductions: Accretion of credit loss impairment due to an increase in expected cash flows	(590)	(109)	(959)	(358)
Balance, end of period	$ 33,358	$ 10,967	$ 33,358	$ 10,967