Incremental Costs to Obtain a Contract with a Customer (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Commission and incentive payments
Capitalized Contract Cost [Line Items]
Total deferred costs	$ 62,993		$ 62,993		$ 58,580
Commission and incentive payments | Deferred costs included in prepaid and other current assets
Capitalized Contract Cost [Line Items]
Total deferred costs	38,929		38,929		35,044
Commission and incentive payments | Deferred costs included in other assets
Capitalized Contract Cost [Line Items]
Total deferred costs	24,064		24,064		$ 23,536
Deferred commissions
Capitalized Contract Cost [Line Items]
Amortization expense	$ 11,200	$ 9,300	$ 32,900	$ 27,300