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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic and Diluted:
Net income (loss) from continuing operations	$ (5,887)	$ (2,444)	$ (15,380)	$ (15,342)
Net income (loss) from discontinued operations	(594)	(7,630)	(7,634)	(21,804)
Net income (loss) attributable to Actua Corporation	$ (6,481)	$ (10,074)	$ (23,014)	$ (37,146)
Basic and Diluted:
Net income (loss) from continuing operations per share (in dollars per share)	$ (0.19)	$ (0.06)	$ (0.49)	$ (0.42)
Net income (loss) from discontinued operations per share (in dollars per share)	(0.02)	(0.21)	(0.24)	(0.59)
Net income (loss) (in dollars per share)	$ (0.21)	$ (0.27)	$ (0.73)	$ (1.01)
Shares used in computation of basic and diluted income (loss) per share (in shares)	30,641	36,776	31,335	36,943