Investment Securities - Changes in Credit Loss Component of Cumulative Other-Than-Temporary Impairment Losses on Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Balance, beginning of period	$ 29,070	$ 10,500	$ 6,119
Additions: Initial credit impairments		19,143	124
Additions: Subsequent credit impairments	5,247	825	4,518
Reductions: Accretion of credit loss impairment due to an increase in expected cash flows	(1,027)	(1,398)	(261)
Balance, end of period	$ 33,290	$ 29,070	$ 10,500