Comprehensive Income - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized holding gains (losses) on available for sale securities before reclassifications	$ 24,471	$ (17,057)	$ 6,422
Amounts reclassified out of accumulated other comprehensive income:
Realized gains on debt securities	(110)	(207)	(961)
Total amounts reclassified out of accumulated other comprehensive income	(110)	(570)	(961)
Unrealized holding gains (losses) on available for sale securities after reclassifications	24,361	(17,627)	5,461
Tax effect	(7,202)	5,193	(2,296)
Unrealized holding gains (losses) on available for sale securities, net of tax	17,159	(12,434)	3,165
Change in unfunded status of the supplemental retirement plans before reclassifications	(6,745)	762	(1,016)
Amounts reclassified out of accumulated other comprehensive income:
Amortization of prior service cost	(54)	(54)	(12)
Amortization of actuarial losses	408	510	390
Total amounts reclassified out of accumulated other comprehensive income	354	(212)	378
Change in unfunded status of the supplemental retirement plans after reclassifications	(6,391)	550	(638)
Tax effect	1,889	(162)	268
Change in unfunded status of the supplemental retirement plans, net of tax	(4,502)	388	(370)
Change in joint beneficiary agreement liability before reclassifications	0	426	(110)
Tax effect	0	0	0
Change in unfunded status of the supplemental retirement plans, net of tax	0	426	(110)
Other comprehensive income (loss)	12,657	(11,620)	2,685
Adoption ASU 2016-01
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	0	62	0
Adoption ASU 2018-02
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	0	(425)	0
Amounts reclassified out of accumulated other comprehensive income:
Total amounts reclassified out of accumulated other comprehensive income	$ 0	$ (668)	$ 0

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

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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