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, 2016	Dec. 31, 2015		Dec. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance, beginning	[1]	$ 24,851	$ 26,246		$ 27,543
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]	(1,124)	(1,177)		(1,297)
Reduction for debt securities called during the period		(6,671)	(218)		0
Balance, ending		$ 17,056	$ 24,851	[1]	$ 26,246	[1]

[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.