Summary of Significant Accounting Policies - Concentration of Credit Risk and Significant Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in the allowance for doubtful accounts
Balance at beginning of year	$ 2,323	$ 1,179	$ 809
Additions (deductions): charged to operations	1,688	2,219	841
Additions (deductions): charged to deferred revenue	194	(391)	(70)
Less: write-offs	1,090	684	401
Balance at end of year	$ 3,115	$ 2,323	$ 1,179