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Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustments out of AOCI [Abstract]
Net securities (gains) losses	$ 388	$ (4,213)	$ 6,341
Interest income (expense)	315,678	311,555	305,629
Other noninterest expense	24,863	24,440	19,597
Income tax (benefit) expense	28,699	34,411	24,436
Net Income	104,388	121,021	112,566
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Total reclassifications, net of tax	1,923	1,117	4,617
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Income tax (benefit) expense	(160)	(204)	(1,827)
Net Income	481	612	5,483
Gains (losses) on AFS securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Net securities (gains) losses	(3)	79	6,622
Amortization of Unrealized Gains Related to Securities Transfer [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Interest income (expense)	644	737	688
Net Unrealized Losses (Gains) on Cash Flow Hedges Reclassified to Interest Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Interest income (expense)	296	(2,012)	(2,300)
Income tax (benefit) expense	(74)	503	575
Net Income	222	(1,509)	(1,725)
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Income tax (benefit) expense	(407)	(672)	(286)
Net Income	1,220	2,014	859
Amortization of Net Losses [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Other noninterest expense	1,535	2,593	1,054
Amortization of Prior Service Costs [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustments out of AOCI [Abstract]
Other noninterest expense	$ 92	$ 93	$ 91

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