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, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Impairment of Investment Securities Disclosure [Abstract]
Balance, beginning (a)	$ 0	$ 16,610	$ 12,208	[1]	$ 17,056	[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
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)	0	219	223		665
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	0	0	(9,164)		0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows	0	0	(2,821)		0
Balance, ending	$ 0	$ 16,391	$ 0		$ 16,391

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