Comprehensive Income - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains (losses) on available for sale securities before reclassifications	$ 15,803	$ 24,471	$ (17,057)
Amounts reclassified out of accumulated other comprehensive income:
Realized gains on debt securities	(7)	(110)	(207)
Total amounts reclassified out of accumulated other comprehensive income	(7)	(110)	(570)
Unrealized holding gains (losses) on available for sale securities after reclassifications	15,796	24,361	(17,627)
Tax effect	(4,670)	(7,202)	5,193
Unrealized holding gains (losses) on available for sale securities, net of tax	11,126	17,159	(12,434)
Change in unfunded status of the supplemental retirement plans before reclassifications	645	(6,745)	762
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(55)	(54)	(54)
Amortization of actuarial losses	9,309	408	510
Total amounts reclassified out of accumulated other comprehensive income	9,254	354	(212)
Change in unfunded status of the supplemental retirement plans after reclassifications	9,899	(6,391)	550
Tax effect	(2,927)	1,889	(162)
Change in unfunded status of the supplemental retirement plans, net of tax	6,972	(4,502)	388
Change in joint beneficiary agreement liability before reclassifications	(596)	0	426
Tax effect	0	0	0
Change in unfunded status of the supplemental retirement plans, net of tax	(596)	0	426
Other comprehensive income (loss)	17,502	12,657	(11,620)
Adoption ASU 2016-01
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	0	0	62
Adoption ASU 2018-02
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	0	0	(425)
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	$ 0	$ 0	$ (668)

Other comprehensive income loss changes in joint beneficiary agreement liability tax.

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