Selected Quarterly Financial Information (Unaudited) (Details) - USD ($) $ / shares in Units, shares 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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Operating expenses
General and administrative	8,867,000	6,186,000	6,208,000	7,213,000	5,760,000	5,313,000	5,909,000	7,058,000	28,474,000	24,040,000	34,634,000
Amortization of intangibles assets	700,000	0	0	0	0	0	0	0	700,000	0	0
Impairment related and other	11,239,000	0	0	0	0	(3,000)	45,000	0	11,239,000	42,000	(85,000)
Total operating expenses	20,806,000	6,186,000	6,208,000	7,213,000	5,760,000	5,310,000	5,954,000	7,058,000	40,413,000	24,082,000	34,549,000
Operating income (loss)	(20,806,000)	(6,186,000)	(6,208,000)	(7,213,000)	(5,760,000)	(5,310,000)	(5,954,000)	(7,058,000)	(40,413,000)	(24,082,000)	(34,549,000)
Other income (loss), net	2,034,000	192,000	2,697,000	957,000	100,000	2,842,000	0	46,000	5,880,000	2,988,000	1,532,000
Interest income	329,000	107,000	128,000	146,000	156,000	24,000	31,000	37,000	710,000	248,000	97,000
Income (loss) from continuing operations before income taxes and noncontrolling interests	(18,443,000)	(5,887,000)	(3,383,000)	(6,110,000)	(5,504,000)	(2,444,000)	(5,923,000)	(6,975,000)	(33,823,000)	(20,846,000)	(32,920,000)
Income tax benefit (expense)	11,489,000	0	0	0	7,296,000	0	0	0	11,489,000	7,296,000	144,000
Income (loss) from continuing operations	(6,954,000)	(5,887,000)	(3,383,000)	(6,110,000)	1,792,000	(2,444,000)	(5,923,000)	(6,975,000)	(22,334,000)	(13,550,000)	(32,776,000)
Income (loss) from discontinued operations, including gain on sale, net of tax	202,880,000	(1,018,000)	(3,943,000)	(4,042,000)	104,091,000	(8,649,000)	(7,635,000)	(8,408,000)	193,877,000	79,399,000	(67,569,000)
Net income (loss)	195,926,000	(6,905,000)	(7,326,000)	(10,152,000)	105,883,000	(11,093,000)	(13,558,000)	(15,383,000)	171,543,000	65,849,000	(100,345,000)
Less: Net income (loss) attributable to the noncontrolling interests	(1,046,000)	(424,000)	(463,000)	(482,000)	(1,348,000)	(1,019,000)	(817,000)	(1,052,000)	(2,415,000)	(4,236,000)	(4,265,000)
Net income (loss) attributable to Actua Corporation	196,972,000	(6,481,000)	(6,863,000)	(9,670,000)	107,231,000	(10,074,000)	(12,741,000)	(14,331,000)	173,958,000	70,085,000	(96,080,000)
Amounts attributable to Actua Corporation:
Net income (loss) from continuing operations	(6,954,000)	(5,887,000)	(3,383,000)	(6,110,000)	1,792,000	(2,444,000)	(5,923,000)	(6,975,000)	(22,334,000)	(13,550,000)	(32,776,000)
Net income (loss) from discontinued operations	$ 203,926,000	$ (594,000)	$ (3,480,000)	$ (3,560,000)	$ 105,439,000	$ (7,630,000)	$ (6,818,000)	$ (7,356,000)	$ 196,292,000	$ 83,635,000	$ (63,304,000)
Basic income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations (in dollars per share)	$ (0.23)	$ (0.19)	$ (0.11)	$ (0.19)	$ 0.05	$ (0.06)	$ (0.16)	$ (0.19)
Income (loss) from discontinued operations (in dollars per share)	6.61	(0.02)	(0.11)	(0.11)	2.94	(0.21)	(0.19)	(0.19)
Net income (loss) (in dollars per share)	$ 6.38	$ (0.21)	$ (0.22)	$ (0.30)	$ 2.99	$ (0.27)	$ (0.35)	$ (0.38)
Shares used in computation of basic income (loss) per share (in shares)	30,869	30,641	31,267	32,113	35,865	36,776	36,760	37,293
Diluted income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations (in dollars per share)	$ (0.23)	$ (0.19)	$ (0.11)	$ (0.19)	$ 0.05	$ (0.06)	$ (0.16)	$ (0.19)
Income (loss) from discontinued operations (in dollars per share)	6.61	(0.02)	(0.11)	(0.11)	2.89	(0.21)	(0.19)	(0.19)
Net income (loss) (in dollars per share)	$ 6.38	$ (0.21)	$ (0.22)	$ (0.30)	$ 2.94	$ (0.27)	$ (0.35)	$ (0.38)
Shares used in computation of diluted income (loss) per share (in shares)	30,869	30,641	31,267	32,113	36,421	36,776	36,760	37,293