Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 192.1	$ 161.2	$ 181.0
Adjustments to reconcile net income from operations to net cash provided by operating activities:
(Reversal of) provision for credit losses	(14.6)	56.9	13.9
Net (gain) loss on disposal of property and equipment	(1.8)	0.3	(1.5)
Depreciation and amortization	44.4	45.1	38.7
Net premium amortization on investment securities	38.8	15.9	8.9
Net gain on investment securities transactions	(1.1)	(0.3)	(0.1)
Realized and unrealized net gains on mortgage banking activities	(26.2)	(49.3)	(30.5)
Net gain on sale of investments in unrelated entities	0.0	(1.0)	0.0
Net gain on sale of OREO	(0.3)	(0.9)	(3.6)
Write-downs of OREO and other assets pending disposal	0.0	0.1	0.9
Mortgage servicing rights impairment (recovery)	(6.9)	9.9	0.4
Deferred taxes	5.0	(6.6)	5.4
Net increase in cash surrender value of company-owned life insurance policies	(6.1)	(7.6)	(6.7)
Stock-based compensation expense	8.9	7.5	8.0
Originations of mortgage loans held for sale	(817.4)	(1,404.2)	(1,015.6)
Proceeds from sales of mortgage loans held for sale	883.9	1,468.4	971.2
Changes in operating assets and liabilities:
Decrease (increase) in interest receivable	3.7	(4.4)	0.3
Increase in other assets	(16.4)	(27.7)	(22.1)
Decrease in interest payable	(2.0)	(6.3)	(13.5)
Decrease (increase) in accounts payable and accrued expenses	(1.7)	11.3	(7.8)
Net cash provided by operating activities	282.3	268.3	127.3
Purchases of investment securities:
Held-to-maturity	(1,238.0)	0.0	0.0
Available-for-sale	(2,717.8)	(2,444.1)	(1,270.0)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	257.6	40.4	35.6
Available-for-sale	1,118.0	1,441.4	978.6
Proceeds from bank-owned life insurance settlements	1.0	5.0	3.2
Extensions of credit to clients, net of repayments	458.5	(901.3)	(81.4)
Recoveries of loans charged-off	9.3	6.7	9.7
Proceeds from sales of OREO	1.7	10.1	25.4
Proceeds from the sale of health savings accounts	0.0	0.0	0.3
Proceeds from sale of investments in unrelated entities	0.0	2.2	0.0
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0.0	0.0	298.4
Capital expenditures, net of proceeds from sales	(10.3)	(30.2)	(16.6)
Net cash used in investing activities	(2,120.0)	(1,869.8)	(16.8)
Cash flows from financing activities:
Net increase in deposits	2,052.6	2,553.5	276.2
Net decrease (increase) in securities sold under repurchase agreements	(40.3)	393.8	(45.2)
Net decrease in other borrowed funds	0.0	0.0	(4.1)
Repayments of long-term debt	0.0	(0.1)	(2.0)
Advances on long-term debt	0.0	98.6	0.1
Proceeds from issuance of common stock	0.4	1.1	1.0
Purchase and retirement of common stock	(5.4)	(116.8)	(2.5)
Dividends paid to common stockholders	(101.6)	(128.6)	(79.2)
Net cash used in financing activities	1,905.7	2,801.5	144.3
Net increase in cash and cash equivalents	68.0	1,200.0	254.8
Cash and cash equivalents at beginning of period	2,276.8	1,076.8	822.0
Cash and cash equivalents at end of period	2,344.8	2,276.8	1,076.8
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(56.8)	(54.4)	(51.2)
Cash paid during the period for interest expense	19.3	32.9	54.7
Supplemental disclosures of noncash investing and financing activities:
Transfer of securities from held-to-maturity to available-for-sale	0.0	0.0	281.1
Non-cash transfer of securities from available-for-sale to held-to-maturity	672.2	0.0	0.0
Right-of-use assets obtained in exchange for operating lease liabilities	5.9	3.6	39.6
Transfer of loans to other real estate owned	0.9	3.3	14.1
Capitalization of internally originated mortgage servicing rights	3.6	11.7	7.3
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	0.0	0.0	78.7
Loans held for sale	0.0	0.0	0.5
Loans	0.0	0.0	416.6
Premises and equipment	0.0	0.0	24.6
Goodwill	0.0	0.0	75.3
Core deposit intangible	0.0	0.0	16.6
Company-owned life insurance	0.0	0.0	15.2
Interest receivable	0.0	0.0	2.2
Other real estate owned	0.0	0.0	2.4
Other assets	0.0	0.0	6.5
Total noncash assets acquired	0.0	0.0	638.6
Deposits	0.0	0.0	706.7
Securities sold under repurchase agreements	0.0	0.0	30.4
Accounts payable and accrued expenses	0.0	0.0	19.9
Long-term debt	0.0	0.0	4.1
Deferred tax liability	0.0	0.0	0.1
Total liabilities assumed	$ 0.0	$ 0.0	$ (761.2)