Goodwill and Intangible Assets - Summary of Intangible Assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite Lived Intangible Assets [Line Items]
Accumulated amortization	$ (4,022)	$ (1,910)
Total	6,936	9,048
Domain names
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 1,268	1,268
Intangible assets, Average Remaining Useful Life	8 months
Trademarks
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 120	120
Intangible assets, Average Remaining Useful Life	59 months
Software
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 620	620
Intangible assets, Average Remaining Useful Life	10 months
Capitalized learning content
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 400	400
Intangible assets, Average Remaining Useful Life	37 months
Trade names
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 320	320
Intangible assets, Average Remaining Useful Life	26 months
Developed technology
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 5,320	5,320
Intangible assets, Average Remaining Useful Life	38 months
Customer relationships
Finite Lived Intangible Assets [Line Items]
Intangible assets, gross	$ 2,910	$ 2,910
Intangible assets, Average Remaining Useful Life	26 months