Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest income:
Interest and fees on loans	$ 470.9	$ 404.3	$ 324.7
Interest and dividends on investment securities:
Taxable	62.3	55.4	42.8
Exempt from federal taxes	2.0	2.4	3.2
Interest on deposits in banks	18.8	11.3	7.1
Total interest income	554.0	473.4	377.8
Interest expense:
Interest on deposits	49.3	32.6	21.4
Interest on securities sold under repurchase agreements	3.9	2.7	1.3
Interest Expense, Short-term Borrowings	0.0	0.2
Interest on other borrowed funds	1.3	1.3	0.5
Interest on long-term debt			1.7
Interest on subordinated debentures held by subsidiary trusts	4.5	4.1	3.1
Total interest expense	59.0	40.9	28.0
Net interest income	495.0	432.5	349.8
Provision for loan losses	13.9	8.6	11.0
Net interest income after provision for loan losses	481.1	423.9	338.8
Non-interest income:
Fees and Commissions, Mortgage Banking and Servicing	30.4	24.9	28.9
Loss on termination of interest rate swap	0.0	0.0	(1.1)
Investment securities gains (losses), net	0.1	(0.1)	0.7
Other income	15.9	15.1	14.3
Total non-interest income	149.9	143.3	141.8
Non-interest expense:
Salaries and wages	155.3	146.4	122.7
Employee benefits	51.5	47.9	37.6
Information Technology and Data Processing	32.3	28.7	25.1
Occupancy, Net	28.3	25.4	22.4
Equipment Expense	13.2	12.7	11.5
FDIC insurance premiums	11.6	10.5	10.4
Federal Deposit Insurance Corporation Premium Expense	3.5	5.6	4.7
Mortgage servicing rights impairment (recovery)	4.3	3.1	3.0
OREO expense, net of income	0.4	0.0	(0.1)
Other Real Estate Owned Expense Net Of Income	(2.2)	0.3	0.4
Core deposit intangibles amortization	11.2	7.9	5.5
Other expenses	66.2	60.0	53.5
Acquisition related expenses	20.3	12.4	27.2
Total non-interest expense	395.9	360.9	323.9
Income before income tax expense	235.1	206.3	156.7
Income tax expense	54.1	46.1	50.2
Net income	$ 181.0	$ 160.2	$ 106.5
Basic earnings per common share (in dollars per share)	$ 2.84	$ 2.77	$ 2.07
Diluted earnings per common share (in dollars per share)	$ 2.83	$ 2.75	$ 2.05
Credit and Debit Card [Member]
Interest expense:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 41.5	$ 43.3	$ 43.3
Financial Service, Other [Member]
Interest expense:
Revenue from Contract with Customer, Excluding Assessed Tax	17.1	15.1	13.3
Investment Advisory, Management and Administrative Service [Member]
Non-interest income:
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions	23.8	23.2	21.1
Deposit Account [Member]
Interest expense:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 21.1	$ 21.8	$ 21.3