Impairment of Investment Securities - Cumulative Roll Forward of Credit Losses Recognized in Earnings for Debt Securities Held and Not Intended to be Sold (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Impairment of Investment Securities Disclosure [Abstract]
Balance, beginning (a)	$ 43,274	[1]	$ 44,736	[1]
Credit losses on debt securities for which other-than-temporary impairment was not previously recognized	0		0
Additional credit losses on debt securities for which other-than-temporary impairment was previously recognized	0		0
Increases in cash flows expected to be collected, recognized over the remaining life of the security (b)	(283)	[2]	(235)	[2]
Balance, ending	$ 42,991		$ 44,501

[1] The beginning balance represents credit related losses included in other-than-temporary impairment charges recognized on debt securities in prior periods.

[2] Represents the increase in cash flows recognized in interest income during the period.