Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest income:
Interest and fees on loans	$ 453.4	$ 470.9	$ 404.3
Interest and dividends on investment securities:
Taxable	63.4	62.3	55.4
Exempt from federal taxes	2.7	2.0	2.4
Interest on deposits in banks	4.1	18.8	11.3
Total interest income	523.6	554.0	473.4
Interest expense:
Interest on deposits	18.1	49.3	32.6
Interest on securities sold under repurchase agreements	0.9	3.9	2.7
Interest on other borrowed funds	0.0	0.0	0.2
Interest on long-term debt	4.6	1.3	1.3
Interest on subordinated debentures held by subsidiary trusts	3.0	4.5	4.1
Total interest expense	26.6	59.0	40.9
Net interest income	497.0	495.0	432.5
Provision for credit losses	56.9	13.9	8.6
Net interest income after provision for credit losses	440.1	481.1	423.9
Non-interest income:
Fees and Commissions, Mortgage Banking and Servicing	47.3	33.2	29.7
Investment securities gains (losses), net	0.3	0.1	(0.1)
Other income	14.5	15.9	15.1
Total non-interest income	156.7	142.6	138.8
Non-interest expense:
Salaries and wages	173.7	155.3	146.4
Employee benefits	49.4	51.5	47.9
Information Technology and Data Processing	32.8	32.3	28.7
Occupancy, Net	28.5	28.3	25.4
Equipment Expense	15.5	13.2	12.7
OREO expense, net of income	(0.5)	(2.2)	0.3
Professional fees	10.9	11.6	10.5
FDIC insurance premiums	5.9	3.5	5.6
Core deposit intangibles amortization	10.9	11.2	7.9
Other expenses	60.4	63.6	58.6
Acquisition related expenses	0.0	20.3	12.4
Total non-interest expense	387.5	388.6	356.4
Income before income tax expense	209.3	235.1	206.3
Income tax expense	48.1	54.1	46.1
Net income	$ 161.2	$ 181.0	$ 160.2
Basic earnings per common share (in dollars per share)	$ 2.53	$ 2.84	$ 2.77
Diluted earnings per common share (in dollars per share)	$ 2.53	$ 2.83	$ 2.75
Credit and Debit Card [Member]
Non-interest income:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 41.1	$ 41.5	$ 43.3
Financial Service, Other [Member]
Non-interest income:
Revenue from Contract with Customer, Excluding Assessed Tax	12.1	7.0	5.8
Investment Advisory, Management and Administrative Service [Member]
Non-interest income:
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions	23.8	23.8	23.2
Deposit Account [Member]
Non-interest income:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 17.6	$ 21.1	$ 21.8