Net Income (Loss) Per Share - Calculations of Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Basic and Diluted:
Net income (loss) from continuing operations	$ (9,670)	$ (12,360)
Net income (loss) from discontinued operations	0	(1,971)
Net income (loss) attributable to Actua Corporation	$ (9,670)	$ (14,331)
Basic and Diluted:
Net income (loss) from continuing operations per share (in dollars per share)	$ (0.30)	$ (0.33)
Net income (loss) from discontinued operations per share (in dollars per share)	0.00	(0.05)
Net income (loss) (in dollars per share)	$ (0.30)	$ (0.38)
Shares used in computation of basic and diluted income (loss) per share (in shares)	32,113	37,293