Net 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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Basic and Diluted:
Net loss from continuing operations	$ (6,467)	$ (11,732)	$ (29,590)	$ (37,390)
Net loss from discontinued operations	(3,607)	(2,150)	(7,556)	(6,539)
Net loss attributable to Actua Corporation	$ (10,074)	$ (13,882)	$ (37,146)	$ (43,929)
Basic and diluted loss per share attributable to Actua Corporation:
Net loss from continuing operations per share (in dollars per share)	$ (0.17)	$ (0.31)	$ (0.80)	$ (1.01)
Net loss from discontinued operations per share (in dollars per share)	(0.10)	(0.06)	(0.21)	(0.18)
Net loss (in dollars per share)	$ (0.27)	$ (0.37)	$ (1.01)	$ (1.19)
Shares used in computation of basic and diluted loss per share (in shares)	36,776	37,146	36,943	37,038