Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Trade accounts receivable, allowance (in dollars)	$ 11.6	$ 11.7
Properties and equipment, accumulated depreciation and amortization	$ 1,482.8	$ 1,463.4
Common stock issued, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock issued, shares authorized	300,000,000	300,000,000
Common stock issued, shares outstanding	66,792,968	67,780,410
Treasury Stock, Shares	10,663,659	9,676,217