Accumulated Other Comprehensive Income (Loss) - Summary of Changes (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 1,875,645	$ 1,786,437
Other comprehensive income (loss) before reclassifications	(31,362)	44,583
Amounts reclassified from accumulated other comprehensive income	(3,585)	(6,989)
Total other comprehensive income (loss), net of tax	(34,947)	37,594
Ending balance	1,868,090	1,833,656
Total
Accumulated Other Comprehensive Income (Loss)
Beginning balance	74,836	27,874
Ending balance	39,889	65,468
Unrealized Gains (Losses) on Securities Available for Sale
Accumulated Other Comprehensive Income (Loss)
Beginning balance	87,988	38,872
Other comprehensive income (loss) before reclassifications	(31,379)	45,729
Amounts reclassified from accumulated other comprehensive income	(4,200)	(7,503)
Total other comprehensive income (loss), net of tax	(35,579)	38,226
Ending balance	52,409	77,098
Unrealized Gains (Losses) on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(1,594)	(1,141)
Other comprehensive income (loss) before reclassifications	17	(1,146)
Amounts reclassified from accumulated other comprehensive income	615	514
Total other comprehensive income (loss), net of tax	632	(632)
Ending balance	(962)	(1,773)
Unrealized Gains (Losses) on Defined Benefit Plans
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(11,558)	(9,857)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income	0	0
Total other comprehensive income (loss), net of tax	0	0
Ending balance	$ (11,558)	$ (9,857)