Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net income	$ 41.6	$ 36.7
Adjustments to reconcile net income to net cash provided by operating activities:
Increase (Decrease) in Mortgage Loans Held-for-sale	129.8	204.1
Provision for Loan and Lease Losses	3.7	2.1
Depreciation and amortization	7.5	6.6
Net premium amortization on investment securities	2.0	2.8
Realized and unrealized net gains on mortgage banking activities	(4.7)	(1.6)
Impairment of Real Estate and Other Assets Pending Disposal	0.3	0.0
Net loss (gain) on sale of OREO	(0.4)	0.0
Deferred income tax expense	2.7	10.1
Net increase in cash surrender value of company-owned life insurance	(0.7)	(1.3)
Stock-based compensation expense	1.9	1.1
Proceeds from sales of mortgage loans held for sale	132.7	218.8
Changes in operating assets and liabilities, net of effects of acquisition:
Increase in interest receivable	(1.6)	(0.9)
(Increase) decrease in other assets	(3.7)	3.9
Increase in accrued interest payable	2.2	0.1
Decrease in accounts payable and accrued expenses	(6.1)	(10.9)
Net cash provided by operating activities	47.6	63.4
Cash flows from investing activities:
Purchase of investment securities, Available-for-sale	(161.3)	(212.9)
Proceeds from maturities and paydowns of held-to-maturity investment securities	11.3	22.2
Proceeds from maturities and paydowns of available-for-sale investment securities	164.0	114.9
Extensions of credit to customers, net of repayments	(3.2)	(53.3)
Recoveries of loans charged-off	1.7	4.5
Proceeds from sale of OREO	1.8	0.3
Capital expenditures, net of sales	(2.2)	(2.4)
Net cash provided by (used in) investing activities	12.1	(126.7)
Cash flows from financing activities:
Proceeds from Issuance of Unsecured Debt	0.0	2.6
Net increase (decrease) in deposits	134.0	91.0
Net (decrease) increase in securities sold under repurchase agreements	(39.8)	(9.2)
Net decrease in other borrowed funds	0.0	(20.0)
Proceeds from issuance of common stock	0.4	0.8
Purchase and retirement of common stock	(2.4)	(0.9)
Dividends paid to common stockholders	(18.7)	(15.7)
Net cash provided by financing activities	73.5	48.6
Net increase (decrease) in cash and cash equivalents	133.2	(14.7)
Cash and cash equivalents at beginning of period	822.0	758.9
Cash and cash equivalents at end of period	955.2	744.2
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	0.0	4.6
Cash paid during the period for interest expense	12.4	7.7
Noncash or Part Noncash Acquisition, Value of Assets Acquired [Abstract]
Amortization of unrealized gains and losses on transfers of securities	0.0	0.5
Right-of-use assets obtained in exchange for operating lease liabilities	54.6	0.0
Transfer of loans to other real estate owned	8.4	1.2
Capitalization of internally originated mortgage servicing rights	$ 1.1	$ 1.3