Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Significant Accounting Policies
Accounts receivable, net	$ 266,660	$ 270,812
Deferred revenues	19,810	16,940
Costs to obtain and fulfill a contract	24,005	18,624
Customer deposits	28,914	34,235
Unbilled receivables, net	38,800	$ 39,500
Revenue recognized from deferred revenue	10,600
Amortization of contract costs	$ 4,800
Practical expedient on incremental costs of obtaining contracts	true