Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Supplemental disclosure of cash flow information:
Interest paid	$ 0.3	$ 0.3
Non-cash investing and financing activities (included in Accounts payable):
Stock repurchases not yet settled	2.0	0.1
Unpaid purchases of property and equipment	3.7	2.0
Cash and cash equivalents	34.4	70.7	$ 55.2
Restricted cash included in Prepaid expenses and other current assets	0.3	0.3
Restricted cash included in Other assets	12.0	10.9
Total cash, cash equivalents and restricted cash shown in the Statements of Consolidated Cash Flows	$ 46.7	$ 81.9	$ 67.7	$ 83.7