Investment Securiities - Debt Securities Credit Loss Recognized in Income, and Other Losses Recorded in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Credit losses on debt securities held:
Balance, January 1	$ 11,355	$ 10,955
Additions related to other-than-temporary losses not previously recognized		400
Balance, December 31	$ 11,355	$ 11,355