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	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016		Sep. 30, 2015
Impairment of Investment Securities Disclosure [Abstract]
Balance, beginning (a)	$ 24,310	$ 25,366	$ 24,851	[1]	$ 26,246	[1]
Credit losses on debt securities for which other-than-temporary impairment was not previously recognized	0	0	0		0
Additional credit losses on debt securities for which other-than-temporary impairment was previously recognized	0	0	0		0
Balance, ending	24,040	25,111	24,040		25,111
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows	0	0	0		218
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)	$ 270	$ 255	$ 811		$ 917

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