Fair Value Measurements - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	May 01, 2020	Aug. 31, 2018	Jan. 12, 2018	Oct. 21, 2016
Class of Warrant or Right [Line Items]
Warrant shares (in shares)	414,286	1,339,286
Change in fair value of warrant	$ (1,826)	$ (141)	$ 368
Hale Capital, LLP
Class of Warrant or Right [Line Items]
Warrant shares (in shares)	314,286	314,286
Warrant, exercise price (in dollars per share)							$ 2.80
ESW Holdings, Inc.
Class of Warrant or Right [Line Items]
Warrant shares (in shares)	0	925,000		925,000		925,000
Warrant, exercise price (in dollars per share)				$ 1.96		$ 1.96
iStudy Co., Ltd.
Class of Warrant or Right [Line Items]
Warrant shares (in shares)	100,000	100,000			100,000
Warrant, exercise price (in dollars per share)					$ 2.43