Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other comprehensive income (loss):
Net income	$ 53,695	$ 46,255	$ 38,471
Available-for-sale investment securities:
Gross unrealized holding gain (loss) arising in the period	19,635	(3,910)	(555)
Related tax (expense) benefit	4,123	821	195
Less: reclassification adjustment for net gain (loss) included in net income	164	(146)	2,983
Related tax (expense) benefit	(34)	31	(1,044)
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2018-02			(370)
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2016-01	0	(5,020)
Net effect on other comprehensive income (loss)	15,382	(7,994)	(2,669)
Defined benefit plans:
Net (loss) gain arising during the period	(385)	325	(616)
Related tax benefit (expense)	81	(69)	216
Amortization of unrecognized gain on service benefit plans	72	99	96
Related tax expense	(15)	(21)	(34)
Recognition of loss due to settlement and curtailment	0	267	242
Related tax expense	0	(56)	(85)
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2018-02			(754)
Net effect on other comprehensive (loss) income	(247)	545	(935)
Net loss arising during the period	(4,591)	(341)	(395)
Related tax benefit	964	72	138
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2018-02			200
Net effect on other comprehensive (loss) income	(3,627)	(269)	(57)
Total other comprehensive income (loss), net of tax	11,508	(7,718)	(3,661)
Total comprehensive income	65,203	38,537	34,810
Income tax expense	11,663	8,686	18,732
Adjustments for New Accounting Pronouncement [Member]
Available-for-sale investment securities:
Amounts reclassified out of accumulated other comprehensive (loss) income per ASU 2016-01		(5,000)
Defined benefit plans:
Income tax expense	$ 700	$ 700	$ 900