Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Statement of Cash Flows [Abstract]
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	$ (0.5)	$ (1.1)
Repayments of Other Long-term Debt	0.4	0.1
Cash flows from operating activities:
Net income	119.8	72.3
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	7.0	7.5
Depreciation and amortization	19.9	17.9
Net premium amortization on investment securities	7.9	8.6
Gain (Loss) on Sale of Debt Investments	0.0	0.8
Realized and unrealized net gains on mortgage banking activities	(4.2)	(15.3)
Net gain on sale of OREO	(0.4)	(0.4)
Asset Impairment Charges	0.1	0.3
Proceeds From The Sale Of Health Savings Accounts	0.0	(3.1)
Mortgage servicing rights recovery	0.0	0.1
Deferred tax expenses	13.2	19.8
Net increase in cash surrender value of company-owned life insurance	(3.6)	(3.8)
Stock-based compensation expense	4.3	3.4
Originations of mortgage loans held for sale	(738.3)	(719.2)
Proceeds from sales of mortgage loans held for sale	747.1	731.1
Changes in operating assets and liabilities, net of effects of acquisition:
Increase in interest receivable	(6.4)	(3.2)
(Increase) decrease in other assets	(0.7)	(4.8)
Increase in accrued interest payable	0.9	0.2
Decrease in accounts payable and accrued expenses	5.3	(15.7)
Net cash provided by operating activities	171.9	94.7
Cash flows from investing activities:
Purchase of investment securities, Held-to-maturity	(0.3)	(12.8)
Purchase of investment securities, Available-for-sale	(320.6)	(386.5)
Proceeds from maturities and paydowns of held-to-maturity investment securities	66.9	76.0
Proceeds from maturities and paydowns of available-for-sale investment securities	333.3	319.9
Extensions of credit to customers, net of repayments	(225.5)	(20.9)
Recoveries of loans charged-off	9.6	6.5
Proceeds from sale of OREO	4.3	4.1
Acquisition of intangible assets	0.0	(28.0)
Proceeds from the sale of Health Savings Accounts	0.0	6.1
Acquisition of bank and bank holding company, net of cash and cash equivalents received	28.2	91.8
Capital expenditures, net of sales	(0.5)	(12.2)
Net cash provided by (used in) investing activities	(104.6)	44.0
Cash flows from financing activities:
Net increase (decrease) in deposits	214.4	(111.6)
Net (decrease) increase in securities sold under repurchase agreements	(7.1)	97.7
Net decrease in other borrowed funds	(24.5)	10.0
Advances on long-term debt	2.7	0.0
Proceeds from issuance of common stock	1.9	2.0
Purchase and retirement of common stock	(1.0)	(0.9)
Dividends paid to common stockholders	(47.2)	(35.0)
Net cash provided by (used in) financing activities	138.8	(37.9)
Net increases (decrease) in cash and cash equivalents	206.1	100.8
Cash and cash equivalents at beginning of period	758.9	782.0
Cash and cash equivalents at end of period	965.0	882.8
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	15.8	24.3
Cash paid during the period for interest expense	27.1	19.9
Noncash or Part Noncash Acquisition, Value of Assets Acquired [Abstract]
Transfer of loans to loans held for sale	1.4	6.3
Transfer of loans to other real estate owned	10.7	3.2
Capitalization of internally originated mortgage servicing rights	4.5	4.1
Noncash or Part Noncash Acquisition, Investments Acquired	3.1	424.3
Noncash Or Part Noncash Acquisition, HTM Investments Acquired	0.0	57.3
Noncash Or Part Noncash Acquisition, Loans Held For Sale Acquired	0.0	10.2
Noncash Or Part Noncash Acquisition, Loans Acquired	713.1	2,080.4
Noncash or Part Noncash Acquisition, Fixed Assets Acquired	14.0	46.7
Noncash Or Part Noncash Acquisition, Goodwill	100.8	232.2
Noncash or Part Noncash Acquisition, Intangible Assets Acquired	15.7	48.0
Noncash Or Part Noncash Acquisition, Mortgage Servicing Asset Acquired	0.0	3.5
Noncash Or Part Noncash Acquisition, Interest Receivable Acquired	3.6	57.0
Noncash Or Part Noncash Acquisition, Company-Owned Life Insurance Acquired	9.5	26.0
Noncash Or Part Noncash Acquisition, Deferred Tax Assets Acquired	0.0	7.6
Noncash Or Part Noncash Acquisition, Other Real Estate Owned Acquired	0.6	1.2
Noncash or Part Noncash Acquisition, Other Assets Acquired	6.5	31.6
Noncash or Part Noncash Acquisition, Value of Assets Acquired	866.9	3,026.0
Noncash Or Part Noncash Acquisition, Deposits Acquired	696.3	2,669.0
Noncash or Part Noncash Acquisition, Debt Assumed	7.0	0.0
Noncash or Part Noncash Acquisition, Other Liabilities Assumed	6.0	0.0
Noncash or Part Noncash Acquisition, Trust Preferred Securities Assumed	4.4	0.0
Noncash or Part Noncash Acquisition, Deferred Tax Liability	0.4	0.0
Noncash or Part Noncash Acquisition, Payables Assumed	7.7	62.9
Noncash or Part Noncash Acquisition, Value of Liabilities Assumed	$ 721.8	$ 2,731.9