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								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic and Diluted:
Net income (loss) from continuing operations	$ 4,380	$ (6,467)	$ (10,763)	$ (12,360)	$ (50,170)	$ (11,732)	$ (13,136)	$ (12,522)	$ (25,210)	$ (87,560)	$ (34,872)
Net income (loss) from discontinued operations	102,851	(3,607)	(1,978)	(1,971)	(1,981)	(2,150)	(2,146)	(2,243)	95,295	(8,520)	11,316
Net income (loss) attributable to Actua Corporation	$ 107,231	$ (10,074)	$ (12,741)	$ (14,331)	$ (52,151)	$ (13,882)	$ (15,282)	$ (14,765)	$ 70,085	$ (96,080)	$ (23,556)
Basic and Diluted:
Net income (loss) from continuing operations (in dollars per share)									$ (0.69)	$ (2.36)	$ (0.94)
Net income (loss) from discontinued operations (in dollars per share)									2.60	(0.23)	0.31
Net income (loss) (in dollars per share)									$ 1.91	$ (2.59)	$ (0.63)
Shares used in computation of basic and diluted income (loss) per share (in shares)									36,672	37,080	37,130