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		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Impairment of Investment Securities Disclosure [Abstract]
Document Period End Date		Dec. 31, 2019
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance, beginning	[1]	$ 0	$ 12,208		$ 17,056
Credit losses on debt securities for which other-than-temporary impairment was not previously recognized		0	0		0
Additional credit losses on debt securities for which other-than- temporary impairment was previously recognized		0	0		0
Increases in cash flows expected to be collected, recognized over the remaining life of the security	[2]	0	(223)		(890)
Reduction for debt securities called during the period	[2]	0	(9,164)		0
Balance, ending		0	0	[1]	12,208	[1]
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Called		$ 0	$ 2,821		$ 3,958

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

[2] Represents the increase in cash flows recognized either as principal payments or interest income during the period.