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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Impairment of Investment Securities Disclosure [Abstract]
Balance, beginning (a)	$ 9,759	$ 16,828	$ 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)	76	218	223		446
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	(9,164)	0	(9,164)		0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Cash Flows	(519)	0	(2,821)		0
Balance, ending	$ 0	$ 16,610	$ 0		$ 16,610

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