Summary of Significant Accounting Policies - Rollforward of Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts
Beginning balance	$ 3,703	$ 2,081	$ 565
Charged to operating expenses	1,217	1,963	1,757
Write offs of accounts receivable, net of recoveries	(289)	(341)	(241)
Ending balance	$ 4,631	$ 3,703	$ 2,081