Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental disclosure of cash flow information:
Interest paid	$ 21.1	$ 17.7	$ 22.1
Non-cash investing and financing activities (included in Accounts payable):
Unpaid purchases of property and equipment	7.4	4.6	10.5
Stock repurchases not yet settled (accrued in accounts payable)	0.1	1.8	0.2
Acquisition of property and equipment through capital leasing arrangements	1.2	0.2	0.0
Cash and cash equivalents	51.1	55.2	72.5
Restricted cash included in Prepaid expenses and other current assets	0.3	0.3	0.3
Restricted cash included in Other assets	12.9	12.2	10.9
Total cash, cash equivalents and restricted cash shown in the Statements of Consolidated Cash Flows	$ 64.3	$ 67.7	$ 83.7	$ 188.0