Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Supplemental disclosure of cash flow information:
Interest paid	$ 0.3	$ 0.3
Non-cash investing and financing activities (included in Accounts payable):
Unpaid purchases of property and equipment	5.2	3.7
Stock repurchases not yet settled	0.5	2.0
Components of cash, cash equivalents and restricted cash:
Cash and cash equivalents	135.0	34.4	$ 51.1
Restricted cash included in Prepaid expenses and other current assets	0.3	0.3
Restricted cash included in Other assets	12.9	12.0
Total cash, cash equivalents and restricted cash shown in the Statements of Consolidated Cash Flows	$ 148.2	$ 46.7	$ 64.3	$ 67.7