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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic and Diluted:
Net income (loss) from continuing operations	$ (6,863)	$ (10,763)	$ (16,533)	$ (23,123)
Net income (loss) from discontinued operations	0	(1,978)	0	(3,949)
Net income (loss) attributable to Actua Corporation	$ (6,863)	$ (12,741)	$ (16,533)	$ (27,072)
Basic and Diluted:
Net income (loss) from continuing operations per share (in dollars per share)	$ (0.22)	$ (0.30)	$ (0.52)	$ (0.62)
Net income (loss) from discontinued operations per share (in dollars per share)	0.00	(0.05)	0.00	(0.11)
Net income (loss) (in dollars per share)	$ (0.22)	$ (0.35)	$ (0.52)	$ (0.73)
Shares used in computation of basic and diluted income (loss) per share (in shares)	31,267	36,760	31,687	37,027