Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Unbilled receivables	$ 88.7	$ 101.9
Minimum
Disaggregation of Revenue [Line Items]
Course of recognition	14 days
Maximum
Disaggregation of Revenue [Line Items]
Course of recognition	21 days