Reclassification Adjustments Out of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Reclassification Adjustment out of AOCI [Abstract]
Net securities (gains) losses	$ 201	$ 180	$ 668	$ (632)
Interest income (expense)	79,178	80,446	158,233	157,627
Other noninterest expense	5,416	3,916	8,173	10,873
Income tax (benefit)	11,995	6,564	23,150	8,279
Net income	40,296	24,713	80,142	35,081
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Total reclassifications during the period, net of tax	351	469	719	890
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Income tax (benefit)	(36)	(42)	(71)	(84)
Net income	107	123	214	251
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)	143	165	285	338
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	81	21	91
Income tax (benefit)	0	(20)	(5)	(23)
Net income	0	61	16	68
Accumulated Defined Benefit Plans Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Income tax (benefit)	(82)	(96)	(163)	(191)
Net income	244	285	489	571
Amortization of Net Losses [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Other noninterest expense	298	358	596	716
Amortization of Prior Service Costs [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of AOCI [Abstract]
Other noninterest expense	$ 28	$ 23	$ 56	$ 46