Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 63,077	$ 49,370	$ 46,249
Change in securities available-for-sale (AFS):
Unrealized holding gains (losses) on available-for-sale securities arising during the period	14,431	8,754	(3,356)
Reclassification adjustment for (gains) losses realized in income	(1,464)	(24)	(173)
Net unrealized gains (losses)	12,967	8,730	(3,529)
Income tax effect	(2,723)	(1,834)	742
Net of tax amount	10,244	6,896	(2,787)
Change in unrealized gain/(loss) on postretirement benefit:
Net gain (loss) on defined benefit postretirement medical plan realized during the period	195	(310)	560
Net amortization and deferral	13	14	18
Net gain (loss) activity during the period	208	(296)	578
Income tax effect	(43)	143	(203)
Net of tax amount	165	(153)	375
Total other comprehensive income (loss)	10,409	6,743	(2,412)
Comprehensive income	$ 73,486	$ 56,113	$ 43,837