Business Combination - Additional Information (Details) $ in Thousands, shares in Millions		12 Months Ended
	Sep. 06, 2017 USD ($) Founder shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Aggregate purchase price, paid			$ 2,959
Goodwill		$ 1,382	1,382
Amortization of intangible assets expense		600	200
Estimated future amortization expenses for intangible asset for 2019		600
Estimated future amortization expenses for intangible asset for 2020		500
Estimated future amortization expenses for intangible asset for 2021		300
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
Acquisition related cost which included in general and administrative expenses			$ 400
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