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Deferred Commissions (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Capitalized Contract Cost [Line Items]      
Deferred commissions   $ 21.9 $ 19.3
Deferred commissions amortization   $ 15.6 $ 11.2
Minimum      
Capitalized Contract Cost [Line Items]      
Sales commissions amortization period (in years) 3 years 3 years 1 year
Maximum      
Capitalized Contract Cost [Line Items]      
Sales commissions amortization period (in years) 7 years 7 years 2 years