Fair Value Measurements - Summary of Reconciliation of Beginning and Ending Balances of Recurring Fair Value Measurements (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Earnout liability:
Balance of earnout liability included in current liabilities	$ 3,000	$ 3,061
Recurring Basis | Significant Unobservable Inputs (Level 3) | Earnout Related to Acquisitions
Earnout liability:
Balance beginning of period	380	6,235
Payments of earnout liability		(2,101)
Settlement of earnout liability		(3,051)
Total unrealized (gains) losses included in other income	(380)	(703)
Balance end of period		380
Balance of earnout liability included in current liabilities	$ 3,000	3,061
Balance of earnout liability included in noncurrent liabilities		$ 370