Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net income	$ 36.7	$ 23.2
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for loan losses	2.1	1.7
Net loss on disposal of premises and equipment	0.0	0.1
Depreciation and amortization	6.6	4.6
Net premium amortization on investment securities	2.8	2.8
Net gain on investment securities transactions	0.0	0.0
Realized and unrealized net gains on mortgage banking activities	(1.6)	(4.9)
Net loss on sale of OREO	0.0	0.2
Gain Loss on Sale of Mortgage Servicing	0.0	0.1
Mortgage servicing rights recovery	0.0	(0.1)
Deferred tax expenses	10.1	2.0
Net increase in cash surrender value of company-owned life insurance	(1.3)	(1.1)
Stock-based compensation expense	1.1	0.9
Originations of mortgage loans held for sale	(204.1)	(193.1)
Proceeds from sales of mortgage loans held for sale	218.8	235.0
Changes in operating assets and liabilities, net of effects of acquisition:
(Increase) decrease in interest receivable	(0.9)	2.1
Decrease in other assets	3.9	1.7
Increase (decrease) in accrued interest payable	0.1	(0.6)
Decrease in accounts payable and accrued expenses	(10.9)	(1.5)
Net cash provided by operating activities	63.4	73.0
Cash flows from investing activities:
Purchase of investment securities, Held-to-maturity	0.0	(0.9)
Purchase of investment securities, Available-for-sale	(212.9)	(113.4)
Proceeds from maturities and paydowns of held-to-maturity investment securities	22.2	22.8
Proceeds from maturities and paydowns of available-for-sale investment securities	114.9	74.5
Extensions of credit to customers, net of repayments	(53.3)	36.0
Recoveries of loans charged-off	4.5	2.1
Proceeds from sale of OREO	0.3	0.7
Acquisition of intangible assets	0.0	(28.0)
Capital expenditures, net of sales	(2.4)	(4.4)
Net cash used in investing activities	(126.7)	(10.6)
Cash flows from financing activities:
Net increase (decrease) in deposits	91.0	(75.9)
Net (decrease) increase in securities sold under repurchase agreements	(9.2)	50.0
Net decrease in other borrowed funds	(20.0)	0.0
Advances on long-term debt	2.6	0.0
Proceeds from issuance of common stock	0.8	1.1
Purchase and retirement of common stock	(0.9)	(0.9)
Dividends paid to common stockholders	(15.7)	(10.8)
Net cash provided by (cash used) in financing activities	48.6	(36.5)
Net increases (decrease) in cash and cash equivalents	(14.7)	25.9
Cash and cash equivalents at beginning of period	758.9	782.0
Cash and cash equivalents at end of period	744.2	807.9
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	4.6	28.8
Cash paid during the period for interest expense	7.7	6.2
Noncash or Part Noncash Acquisition, Value of Assets Acquired [Abstract]
Transfer of loans to loans held for sale	0.5	0.0
Transfer of loans to other real estate owned	1.2	0.4
Capitalization of internally originated mortgage servicing rights	$ 1.3	$ 1.5