Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other comprehensive income (loss):
Net income	$ 34,767	$ 53,695	$ 46,255
Available-for-sale investment securities:
Gross unrealized holding gain (loss) arising in the period	11,394	19,635	(3,910)
Related tax (expense) benefit	2,393	(4,123)	821
Reclassification adjustment for net loss (gain) included in net income	368	(164)	146
Related tax (benefit) expense	(77)	34	(31)
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2016-01	0	0	(5,020)
Net effect on other comprehensive income (loss)	9,292	15,382	(7,994)
Defined benefit plans:
Net (loss) gain arising during the period	(1,072)	(385)	325
Related tax benefit (expense)	225	81	(69)
Amortization of unrecognized gain on service benefit plans	127	72	99
Related tax expense	(27)	(15)	(21)
Recognition of gain due to settlement and curtailment	1,054	0	267
Related tax expense	(221)	0	(56)
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2018-02			0
Net effect on other comprehensive income (loss)	86	(247)	545
Net loss arising during the period	(8,376)	(4,591)	(341)
Related tax benefit	1,759	964	72
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2018-02			0
Net effect on other comprehensive (loss) income	(6,617)	(3,627)	(269)
Total other comprehensive income (loss), net of tax	2,761	11,508	(7,718)
Total comprehensive income	37,528	65,203	38,537
Income tax expense	$ 7,879	$ 11,663	$ 8,686