Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Changes in accumulated other comprehensive income by component
Balance beginning of the period	$ 1,353,285	$ 1,257,638
Other comprehensive income before reclassification adjustments	7,748	4,452
Amounts reclassified from accumulted other comprehensive income	286	209
Other comprehensive income	8,034	4,661
Balance end of the period	1,341,792	1,316,276
Unrealized holding gains (losses), tax	3,277	1,781
Change in fair value of interest rate swaps, tax	162	0
Realized gain on securities (gain on sales of investments, net)	51
Reclassification adjustment for gains included in net income, tax	14	0
Reclassification adjustment for prior period service costs included in net income, net of tax	581	581
Reclassification adjustment for defined benefit plan loss included in net income, net of tax	(929)	(872)
Reclassification adjustment for prior period service costs included in net income, tax	(99)	(82)
Unrealized gains and (losses) on securities available- for-sale
Changes in accumulated other comprehensive income by component
Balance beginning of the period	3,147	(6,832)
Other comprehensive income before reclassification adjustments	8,157	4,452
Amounts reclassified from accumulted other comprehensive income	37	0
Other comprehensive income	8,194	4,452
Balance end of the period	11,341	(2,380)
Change in fair value of interest rate swaps
Changes in accumulated other comprehensive income by component
Balance beginning of the period	0	0
Other comprehensive income before reclassification adjustments	(409)	0
Amounts reclassified from accumulted other comprehensive income	0	0
Other comprehensive income	(409)	0
Balance end of the period	(409)	0
Change in defined benefit pension plans
Changes in accumulated other comprehensive income by component
Balance beginning of the period	(40,088)	(32,864)
Other comprehensive income before reclassification adjustments	0	0
Amounts reclassified from accumulted other comprehensive income	249	209
Other comprehensive income	249	209
Balance end of the period	(39,839)	(32,655)
Total
Changes in accumulated other comprehensive income by component
Balance beginning of the period	(36,941)	(39,696)
Other comprehensive income	8,034	4,661
Balance end of the period	$ (28,907)	$ (35,035)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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