Consolidated Statements of Cash Flows - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net income	$ 66.0	$ 79.5
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for credit losses	48.5	7.5
Net gain on disposal of premises and equipment	(0.1)	0.0
Depreciation and amortization	21.2	19.3
Net premium amortization on investment securities	5.7	4.2
Realized and unrealized net gains on mortgage banking activities	(24.9)	(11.9)
Net gain on sale of investments in unrelated entities	(1.0)	0.0
Net gain on sale of OREO	(0.5)	(0.5)
Write-downs of OREO and other assets pending disposal	0.0	0.4
Mortgage servicing rights impairment	8.4	0.0
Deferred taxes	5.8	(2.7)
Net decrease (increase) in cash surrender value of company-owned life insurance	0.7	(1.4)
Stock-based compensation expense	3.8	4.6
Originations of mortgage loans held for sale	(757.3)	(474.4)
Proceeds from sales of mortgage loans held for sale	706.7	453.2
Changes in operating assets and liabilities, net of effects of acquisition:
Increase in interest receivable	(9.8)	(3.1)
Increase in other assets	(49.6)	(15.9)
Decrease in accrued interest payable	(3.3)	(13.5)
Increase (decrease) in accounts payable and accrued expenses	0.5	(2.6)
Net cash provided by operating activities	9.2	48.1
Purchases of investment securities:
Available-for-sale	(1,029.0)	(294.6)
Proceeds from maturities and pay-downs of investment securities:
Held-to-maturity	24.3	29.4
Available-for-sale	739.9	356.5
Extensions of credit to clients, net of repayments	(1,111.8)	(133.1)
Recoveries of loans charged-off	3.3	5.3
Proceeds from sale of OREO	3.7	2.6
Proceeds from the sale of Health Savings Accounts	0.0	0.3
Proceeds from sale of investments in unrelated entities	2.2	0.0
Acquisition of bank and bank holding company, net of cash and cash equivalents received	0.0	298.4
Capital expenditures, net of sales	(12.5)	(7.7)
Net cash (used in) provided by investing activities	(1,379.9)	257.1
Cash flows from financing activities:
Net increase in deposits	1,676.9	102.5
Net increase (decrease) in securities sold under repurchase agreements	58.5	(56.1)
Net decrease in other borrowed funds	0.0	(4.1)
Repayments of long-term debt	(0.1)	(0.1)
Advances on long-term debt	98.5	0.1
Proceeds from issuance of common stock	0.7	0.7
Purchase and retirement of common stock	(32.6)	(2.5)
Dividends paid to common stockholders	(83.0)	(38.8)
Net cash provided by financing activities	1,718.9	1.7
Net increase in cash and cash equivalents	348.2	306.9
Cash and cash equivalents at beginning of period	1,076.8	822.0
Cash and cash equivalents at end of period	1,425.0	1,128.9
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	24.5	24.5
Cash paid during the period for interest expense	18.7	26.8
Supplemental disclosures of non-cash investing and financing activities:
Right-of-use assets obtained in exchange for operating lease liabilities	3.3	54.6
Transfer of loans to other real estate owned	1.2	13.3
Capitalization of internally originated mortgage servicing rights	6.3	2.8
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	0.0	78.7
Loans held for sale	0.0	0.5
Loans held for investment	0.0	416.6
Premises and equipment	0.0	22.5
Goodwill	0.0	77.2
Core deposit intangible	0.0	16.6
Company-owned life insurance	0.0	15.2
Interest receivable	0.0	2.2
Other real estate owned	0.0	2.4
Other assets	0.0	6.5
Total noncash assets acquired	0.0	638.4
Deposits	0.0	706.7
Securities sold under repurchase agreements	0.0	30.4
Accounts payable and accrued expenses	0.0	19.4
Long-term debt	0.0	4.1
Deferred tax liability	0.0	0.1
Total liabilities assumed	$ 0.0	$ 760.7