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	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Impairment of Investment Securities Disclosure [Abstract]
Balance, beginning	$ 27,543	[1]	$ 43,274	[1]	$ 44,736	[1]
Credit losses on debt securities for which other-than-temporary impairment was not previously recognized	0		0		0
Additional credit losses on debt securities for which other-than- temporary impairment was previously recognized	0		0		0
Increases in cash flows expected to be collected, recognized over the remaining life of the security	(1,297)	[2]	(2,375)	[2]	(1,462)	[2]
Reduction for debt securities called during the period	0		(13,356)		0
Balance, ending	$ 26,246		$ 27,543	[1]	$ 43,274	[1]

[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 either as principal payments or interest income during the period.