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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic and Diluted:
Income (loss) from continuing operations	$ (52,151)	$ (13,882)	$ (15,282)	$ (14,765)	$ (69)	$ (13,106)	$ (14,209)	$ (10,198)	$ (96,080)	$ (37,582)	$ (25,669)
Income (loss) from discontinued operations	0	0	0	0	10,237	2,426	1,315	48	0	14,026	234,728
Net income (loss) attributable to Actua Corporation	$ (52,151)	$ (13,882)	$ (15,282)	$ (14,765)	$ 10,168	$ (10,680)	$ (12,894)	$ (10,150)	$ (96,080)	$ (23,556)	$ 209,059
Basic and Diluted (usd per share):
Income (loss) from continuing operations (in dollars per share)	$ (1.40)	$ (0.37)	$ (0.41)	$ (0.40)	$ 0.00	$ (0.35)	$ (0.38)	$ (0.27)	$ (2.59)	$ (1.01)	$ (0.70)
Income (loss) from discontinued operations (in dollars per share)	0.00	0.00	0.00	0.00	0.28	0.06	0.04	0.00	0.00	0.38	6.42
Net income (loss) attributable to Actua Corporation per share (in dollars per share)	$ (1.40)	$ (0.37)	$ (0.41)	$ (0.40)	$ 0.28	$ (0.29)	$ (0.35)	$ (0.27)	$ (2.59)	$ (0.63)	$ 5.72
Shares used in computation of basic and diluted income (loss) per share (in shares)	37,190	37,146	37,123	36,842	36,780	37,335	37,313	37,096	37,080	37,130	36,536