Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Significant Accounting Policies
Accounts receivable, net	$ 260,438	$ 270,812
Deferred revenues	17,782	16,940
Costs to obtain and fulfill a contract	24,212	18,624
Customer deposits	30,161	34,235
Unbilled receivables, net	37,600	$ 39,500
Revenue recognized from deferred revenue	12,800
Amortization of contract costs	$ 8,400
Practical expedient on incremental costs of obtaining contracts	true