v3.8.0.1

Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information (Unaudited)
Revenue	$ 86,678	$ 70,250	$ 64,995	$ 64,829	$ 57,350	$ 51,960	$ 49,110	$ 47,444		$ 286,752	$ 205,864	$ 150,194
Costs and expenses
Curriculum and teaching	4,817	1,792								6,609
Servicing and support	13,445	12,939	13,458	10,925	10,859	10,351	10,260	9,512		50,767	40,982	32,047
Technology and content development	12,846	12,735	11,140	9,205	8,496	8,670	8,842	7,275		45,926	33,283	27,211
Marketing and sales	37,700	41,311	37,242	34,670	27,306	28,165	27,483	23,656		150,923	106,610	82,911
General and administrative	17,844	17,227	13,930	13,664	13,061	11,569	10,944	10,447		62,665	46,021	34,123
Total costs and expenses	86,652	86,004	75,770	68,464	59,722	58,755	57,529	50,890		316,890	226,896	176,292
Loss from operations	26	(15,754)	(10,775)	(3,635)	(2,372)	(6,795)	(8,419)	(3,446)		(30,138)	(21,032)	(26,098)
Interest income	104	18	53	196	163	37	91	92		371	383	167
Interest expense	(50)	(36)	(1)				(9)	(26)		(87)	(35)	(552)
Other income (expense), net	106	59	(1,031)							(866)		(250)
Loss before income taxes	186	(15,713)	(11,754)	(3,439)	(2,209)	(6,758)	(8,337)	(3,380)		(30,720)	(20,684)	(26,733)
Income tax benefit (expense)	323	974								1,297
Net loss	$ 509	$ (14,739)	$ (11,754)	$ (3,439)	$ (2,209)	$ (6,758)	$ (8,337)	$ (3,380)	$ (29,423)	$ (29,423)	$ (20,684)	$ (26,733)
Net income (loss) per share, basic (in dollars per share)	$ 0.01	$ (0.30)	$ (0.25)	$ (0.07)
Net income (loss) per share, diluted (in dollars per share)	$ 0.01	$ (0.30)	$ (0.25)	$ (0.07)
Weighted-average shares used in computing net income (loss) per share, basic (in shares)	52,330,067	48,961,914	47,668,397	47,237,341
Weighted-average shares used in computing net income (loss) per share, diluted (in shares)	56,593,108	48,961,914	47,668,397	47,237,341
Net loss per share, basic and diluted (in dollars per share)					$ (0.05)	$ (0.14)	$ (0.18)	$ (0.07)		$ (0.60)	$ (0.44)	$ (0.63)
Weighted-average shares used in computing net loss per share, basic and diluted (in shares)					47,075,167	46,903,628	46,494,464	45,953,082		49,062,611	46,609,751	42,420,356

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