Net Income (Loss) per Share - Calculations (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic and Diluted:
Net income (loss) from continuing operations	$ (6,954)	$ (5,887)	$ (3,383)	$ (6,110)	$ 1,792	$ (2,444)	$ (5,923)	$ (6,975)	$ (22,334)	$ (13,550)	$ (32,776)
Net income (loss) from discontinued operations	203,926	(594)	(3,480)	(3,560)	105,439	(7,630)	(6,818)	(7,356)	196,292	83,635	(63,304)
Net income (loss) attributable to Actua Corporation	$ 196,972	$ (6,481)	$ (6,863)	$ (9,670)	$ 107,231	$ (10,074)	$ (12,741)	$ (14,331)	$ 173,958	$ 70,085	$ (96,080)
Basic and Diluted:
Net income (loss) from continuing operations per share (in dollars per share)									$ (0.72)	$ (0.37)	$ (0.89)
Net income (loss) from discontinued operations per share (in dollars per share)									6.29	2.28	(1.70)
Net income (loss) (in dollars per share)									$ 5.57	$ 1.91	$ (2.59)
Shares used in computation of basic and diluted income (loss) per share (in shares)									31,218	36,672	37,080