Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (loss) Pre-Tax and After-Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Unrealized holding gains (losses) arising during the period, Pre-tax	$ (989)	$ 1,207	$ (11,511)	$ 9,248
Less: Reclassification adjustment for (gains) losses realized in net income, Pre-tax	10	(803)	2,807	(2,450)
Other comprehensive income (loss), Pre-tax	(979)	404	(8,704)	6,798
Reclassification adjustments to retained earnings, Pre-tax			5,830
Change in accumulated other comprehensive income (loss), Pre-tax	(979)	404	(2,874)	6,798
Unrealized holding gains (losses) arising during the period, Tax	(244)	461	(2,751)	3,535
Less: Reclassification adjustment for (gains) losses realized in net income, Tax	2	(308)	683	(938)
Other comprehensive income (loss), Tax	(242)	153	(2,068)	2,597
Reclassification adjustments to retained earnings, Tax			2,811
Change in accumulated other comprehensive income (loss), Tax	(242)	153	743	2,597
Unrealized holding gains (losses) arising during the period, After-tax	(745)	746	(8,760)	5,713
Less: Reclassification adjustment for (gains) losses realized in net income, After-tax	8	(495)	2,124	(1,512)
Other comprehensive income (loss), After-tax	(737)	251	(6,636)	4,201
Reclassification adjustments to retained earnings, After-tax			3,019
Change in accumulated other comprehensive income (loss), After-tax	$ (737)	$ 251	$ (3,617)	$ 4,201