Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts - Reconciliation:
Accounts receivable allowance, Beginning balance	$ 3,532	$ 2,578	$ 2,177
Additions and adjustments	7,680	3,165	1,368
Write-offs	(3,734)	(2,211)	(967)
Accounts receivable allowance, Ending balance	7,478	3,532	2,578
Bad debt expense recognized	$ 1,400	$ 800	$ 700