Investment Securities - 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
	Mar. 31, 2013	Mar. 31, 2012
Investments Debt And Equity Securities [Abstract]
Balance, beginning of period	$ 33,290	$ 29,070
Accretion of credit loss impairment due to an increase in expected cash flows	(113)	(303)
Balance, end of period	$ 33,177	$ 28,767