Comprehensive Income - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Other Comprehensive Income (Loss)
Unrealized holding gains (losses) on available for sale securities before reclassifications	$ 9,553	$ (5,676)	$ 22,263	$ (20,941)
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income		0		(363)
Unrealized holding gains (losses) on available for sale securities after reclassifications	9,553	(5,676)	22,263	(21,304)
Tax effect	(2,824)	1,678	(6,582)	6,280
Unrealized holding gains (losses) on available for sale securities, net of tax	6,729	(3,998)	15,681	(15,024)
Change in unfunded status of the supplemental retirement plans before reclassifications	(89)	0	(177)	668
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(13)	(13)	(27)	(27)
Amortization of actuarial losses	102	127	204	254
Total amounts reclassified out of accumulated other comprehensive income	89	114	177	(441)
Change in unfunded status of the supplemental retirement plans after reclassifications		114		227
Tax effect		(34)		(67)
Change in unfunded status of the supplemental retirement plans, net of tax		80		160
Other comprehensive income (loss)	$ 6,729	(3,918)	$ 15,681	(14,864)
ASU 2016-01 [Member]
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income		0		62
ASU 2018-02 [Member]
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income		0		(425)
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income		$ 0		$ (668)