Acquisitions - 2017 Acquisitions summary of Fair Value of the Consideration (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill		$ 24,673	$ 9,024
Cogeon GmbH
Business Acquisition [Line Items]
Initial cash consideration	$ 12,717
Net working capital adjustment	53
Escrow	2,244
Total fair value of purchase consideration	15,014
Net tangible assets	60
Total acquired intangible assets	5,930
Total identifiable assets acquired	5,990
Goodwill	9,024
Trade names | Cogeon GmbH
Business Acquisition [Line Items]
Total acquired intangible assets	50
Domain names | Cogeon GmbH
Business Acquisition [Line Items]
Total acquired intangible assets	230
Non-compete agreements | Cogeon GmbH
Business Acquisition [Line Items]
Total acquired intangible assets	70
Developed technologies and content library | Cogeon GmbH
Business Acquisition [Line Items]
Total acquired intangible assets	5,510
Content Library | Cogeon GmbH
Business Acquisition [Line Items]
Total acquired intangible assets	$ 70