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
		Mar. 31, 2018	Mar. 31, 2017
Impairment of Investment Securities Disclosure [Abstract]
Balance, beginning (a)	[1]	$ 12,208	$ 17,056
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		9,759	16,828
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows		(2,302)	0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)		$ 147	$ 228

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