Significant Accounting Policies - Deferred Contract Costs and Interest Expense, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Deferred contract costs	$ 32.9	$ 25.7	$ 25.5
Minimum
Finite-Lived Intangible Assets [Line Items]
Amortization period	5 years
Maximum
Finite-Lived Intangible Assets [Line Items]
Amortization period	10 years