Comprehensive Income - Components of Other Comprehensive Income (Loss) and Related Tax Effects (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Equity [Abstract]
Unrealized holding gains on available for sale securities before reclassifications	$ 34,959	$ 9,553	$ 5,398	$ 22,263
Tax effect	(10,334)	(2,824)	(1,595)	(6,582)
Unrealized holding gains on available for sale securities, net of tax	24,625	6,729	3,803	15,681
Change in unfunded status of the supplemental retirement plans before reclassifications	661	(88)	1,109	(177)
Amounts reclassified out of accumulated other comprehensive income (loss):
Amortization of prior service cost	(13)	(14)	(27)	(27)
Amortization of actuarial losses	478	102	956	204
Total amounts reclassified out of accumulated other comprehensive income	465	88	929	177
Change in unfunded status of the supplemental retirement plans after reclassifications	1,126	0	2,038	0
Tax effect	0	0	0	0
Change in unfunded status of the supplemental retirement plans, net of tax	1,126	0	2,038	0
Other comprehensive income	$ 25,751	$ 6,729	$ 5,841	$ 15,681

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