Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Supplemental disclosure of cash flow information:
Interest paid	$ 10.7	$ 10.4
Non-cash investing and financing activities (included in Accounts payable):
Unpaid purchases of property and equipment	5.1	4.1
Stock repurchases not yet settled	0.6	0.6
Acquisition of property and equipment through capital leasing arrangements	0.3	0.3
Components of cash, cash equivalents and restricted cash:
Cash and cash equivalents	164.0	57.1	$ 51.1
Restricted cash included in Prepaid expenses and other current assets	0.3	0.3
Restricted cash included in Other assets	14.5	13.0
Total cash, cash equivalents and restricted cash shown in the Statements of Consolidated Cash Flows	$ 178.8	$ 70.4	$ 64.3	$ 67.7