Business Combinations - Additional Information (Details) $ in Thousands, shares in Millions		3 Months Ended	9 Months Ended
	Sep. 06, 2017 USD ($) Founder shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Aggregate purchase price, paid			$ 2,959
Goodwill		$ 1,382		$ 1,382
Denmark
Business Acquisition [Line Items]
Date of acquisition	Sep. 06, 2017
Aggregate purchase price	$ 3,500
Aggregate purchase price, paid	$ 3,000	$ 500
Number of founders who received shares | Founder	2
Repurchase period for shares issued under business acquisition	3 years
Goodwill	$ 1,400
Identifiable intangible assets acquired	$ 2,200
Identifiable intangible assets, expected useful life	4 years
Denmark | Class B Common Stock
Business Acquisition [Line Items]
Shares of common stock issued | shares	0.1