Net Income (Loss) Per Share Attributable to Common Stockholders - Calculation of Basic and Diluted Net Loss Per Share (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, 2016	Dec. 31, 2015
Numerator:
Loss from continuing operations, net of income taxes	$ (63)	$ (3,273)	$ (6,803)	$ (9,561)	$ (3,094)	$ (4,114)	$ (6,698)	$ (9,944)	$ (19,700)	$ (23,850)	$ (29,176)
Total income (loss) from discontinued operations, net of income taxes	(844)	15,567	4,503	2,701	2,693	497	843	(1,130)	21,927	2,903	(14,054)
Net income (loss)	$ (907)	$ 12,294	$ (2,300)	$ (6,860)	$ (401)	$ (3,617)	$ (5,855)	$ (11,074)	$ 2,227	$ (20,947)	$ (43,230)
Denominator:
Weighted-average number of shares of common stock outstanding for basic and diluted net loss per share attributable to common stockholders	51,195,402	50,642,344	50,205,913	49,998,547	51,644,295	52,473,601	52,633,054	52,372,857	50,511,366	52,279,738	51,684,397
Basic and diluted net income (loss) per share:
Loss from continuing operations (usd per share)	$ 0.00	$ (0.06)	$ (0.14)	$ (0.19)	$ (0.06)	$ (0.08)	$ (0.13)	$ (0.19)	$ (0.39)	$ (0.46)	$ (0.57)
Income (loss) from discontinued operations (usd per share)	(0.02)	0.30	0.09	0.05	0.05	0.01	0.02	(0.02)	0.43	0.06	(0.27)
Net income (loss) (usd per share)	$ (0.02)	$ 0.24	$ (0.05)	$ (0.14)	$ (0.01)	$ (0.07)	$ (0.11)	$ (0.21)	$ 0.04	$ (0.40)	$ (0.84)