Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Supplemental Cash Flow Elements [Abstract]
Interest paid	$ 10.7	$ 10.7
Supplemental disclosure of cash flow information:
Unpaid purchases of property and equipment	2.4	5.1
Stock repurchases not yet settled	0.3	0.6
Non-cash investing and financing activities:
Operating lease liabilities arising from obtaining operating lease assets	0.9
Cash paid for amounts included in the measurement of operating lease liabilities	1.8
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	0.5	0.3
Components of cash, cash equivalents and restricted cash:
Cash and cash equivalents	122.4	164.0	$ 125.6
Restricted cash included in Prepaid expenses and other current assets	0.3	0.3
Restricted cash included in Other assets	14.2	14.5
Total cash, cash equivalents and restricted cash shown in our Statements of Consolidated Cash Flows	$ 136.9	$ 178.8	$ 139.6	$ 64.3