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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic and Diluted:
Income (loss) from continuing operations	$ (13,882)	$ (13,106)	$ (43,929)	$ (37,513)
Income (loss) from discontinued operations	0	2,426	0	3,789
Net income (loss) attributable to Actua Corporation	$ (13,882)	$ (10,680)	$ (43,929)	$ (33,724)
Income (loss) from continuing operations per share (usd per share)	$ (0.37)	$ (0.35)	$ (1.19)	$ (1.01)
Income (loss) from discontinued operations per share (usd per share)	0.00	0.06	0.00	0.10
Net income (loss) attributable to Actua Corporation per share (usd per share)	$ (0.37)	$ (0.29)	$ (1.19)	$ (0.91)
Shares used in computation of basic and diluted income (loss) per share	37,146	37,335	37,038	37,248