Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reclassification Adjustment out of AOCI [Abstract]
Net securities (gains) losses	$ (100)	$ 84	$ 568	$ (548)
Interest income (expense)	77,674	77,943	235,907	235,570
Other noninterest expense	6,232	3,857	14,405	14,730
Income tax (benefit)	11,043	10,988	34,193	19,267
Net income	37,433	35,113	117,575	70,194
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Total reclassifications during the period, net of tax	369	480	1,088	1,370
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Income tax (benefit)	(38)	(39)	(109)	(123)
Net income	111	118	325	369
Gains on AFS Securities [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Net securities (gains) losses	0	0	0	(3)
Amortization of Unrealized Gains Related to Securities Transfer [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Interest income (expense)	149	157	434	495
Net Unrealized Losses on Cash Flow Hedges Reclassified to Interest Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Interest income (expense)	0	101	21	192
Income tax (benefit)	0	(25)	(5)	(48)
Net income	0	76	16	144
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Income tax (benefit)	(86)	(95)	(249)	(286)
Net income	258	286	747	857
Amortization of Net Losses [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Other noninterest expense	317	359	913	1,075
Amortization of Prior Service Costs [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Other noninterest expense	$ 27	$ 22	$ 83	$ 68