Summary of Significant Accounting Policies - Concentration of Credit Risk and Significant Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Changes in the allowance for doubtful accounts
Balance at Beginning of Year	$ 3,115	$ 2,323	$ 1,179
Additions (Deductions): Charged to Operations	1,255	1,688	2,219
Additions (Deductions): Charged to Deferred Revenue	1,177	194	(391)
Less: Write-offs	898	1,090	684
Balance at End of Year	$ 4,649	$ 3,115	$ 2,323