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, 2015		Mar. 31, 2014
Impairment of Investment Securities Disclosure [Abstract]
Balance, beginning (a)	$ 26,246	[1]	$ 27,543	[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
Balance, ending	25,707		27,254
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows	218		0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)	$ 321		$ 289

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