Consolidated Statements of Cash Flows - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net income	$ 78.4	$ 45.0
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	5.0	4.1
Net loss on disposal of premises and equipment	0.0	0.1
Depreciation and amortization	13.1	9.7
Net premium amortization on investment securities	5.5	5.7
Realized and unrealized net gains on mortgage banking activities	(3.0)	(9.9)
Net gain on sale of OREO	(0.4)	(0.2)
Asset Impairment Charges	0.0	0.1
Proceeds From The Sale Of Health Savings Accounts	0.0	(3.4)
Mortgage servicing rights recovery	0.0	(0.1)
Deferred tax expenses	9.3	(15.8)
Net increase in cash surrender value of company-owned life insurance	(2.1)	(2.6)
Stock-based compensation expense	2.9	2.4
Originations of mortgage loans held for sale	(382.5)	(428.3)
Proceeds from sales of mortgage loans held for sale	379.1	478.2
Changes in operating assets and liabilities, net of effects of acquisition:
Increase in interest receivable	(1.3)	(6.0)
(Increase) decrease in other assets	(5.8)	26.2
Increase in accrued interest payable	0.7	3.5
Decrease in accounts payable and accrued expenses	(1.2)	(20.2)
Net cash provided by operating activities	97.7	88.5
Cash flows from investing activities:
Purchase of investment securities, Held-to-maturity	0.0	(2.9)
Purchase of investment securities, Available-for-sale	(230.2)	(201.8)
Proceeds from maturities and paydowns of held-to-maturity investment securities	43.7	43.5
Proceeds from maturities and paydowns of available-for-sale investment securities	228.8	170.2
Extensions of credit to customers, net of repayments	(158.5)	(38.5)
Recoveries of loans charged-off	7.2	3.6
Proceeds from sale of OREO	3.0	1.7
Acquisition of intangible assets	0.0	(28.0)
Proceeds from the sale of Health Savings Accounts	0.0	6.5
Acquisition of bank and bank holding company, net of cash and cash equivalents received	0.0	91.8
Capital expenditures, net of sales	0.1	(7.5)
Net cash provided by (used in) investing activities	(105.9)	38.6
Cash flows from financing activities:
Net increase (decrease) in deposits	10.6	(25.1)
Net (decrease) increase in securities sold under repurchase agreements	(1.2)	42.2
Net decrease in other borrowed funds	(20.0)	15.0
Advances on long-term debt	2.6	0.1
Proceeds from issuance of common stock	1.7	0.9
Purchase and retirement of common stock	(0.9)	(0.9)
Dividends paid to common stockholders	(31.5)	(21.5)
Net cash provided by (used in) financing activities	(38.7)	10.7
Net increases (decrease) in cash and cash equivalents	(46.9)	137.8
Cash and cash equivalents at beginning of period	758.9	782.0
Cash and cash equivalents at end of period	712.0	919.8
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	9.7	14.4
Cash paid during the period for interest expense	16.4	8.6
Noncash or Part Noncash Acquisition, Value of Assets Acquired [Abstract]
Transfer of loans to loans held for sale	0.9	5.8
Transfer of loans to other real estate owned	7.4	1.6
Capitalization of internally originated mortgage servicing rights	2.7	2.7
Noncash or Part Noncash Acquisition, Investments Acquired	0.0	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	10.3
Noncash Or Part Noncash Acquisition, Loans Acquired	0.0	2,080.0
Noncash or Part Noncash Acquisition, Fixed Assets Acquired	0.0	47.9
Noncash Or Part Noncash Acquisition, Goodwill	0.0	231.5
Noncash or Part Noncash Acquisition, Intangible Assets Acquired	0.0	48.0
Noncash Or Part Noncash Acquisition, Mortgage Servicing Asset Acquired	0.0	3.5
Noncash Or Part Noncash Acquisition, Interest Receivable Acquired	0.0	0.0
Noncash Or Part Noncash Acquisition, Company-Owned Life Insurance Acquired	0.0	0.0
Noncash Or Part Noncash Acquisition, Deferred Tax Assets Acquired	0.0	0.0
Noncash Or Part Noncash Acquisition, Other Real Estate Owned Acquired	0.0	1.2
Noncash or Part Noncash Acquisition, Other Assets Acquired	0.0	121.7
Noncash or Part Noncash Acquisition, Value of Assets Acquired	0.0	3,025.7
Noncash Or Part Noncash Acquisition, Deposits Acquired	0.0	2,669.0
Noncash or Part Noncash Acquisition, Payables Assumed	0.0	62.6
Noncash or Part Noncash Acquisition, Value of Liabilities Assumed	$ 0.0	$ 2,731.6