CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0
Operating expenses
General and administrative	28,474,000	24,040,000	34,634,000
Amortization of intangibles assets	700,000	0	0
Impairment related and other	11,239,000	42,000	(85,000)
Total operating expenses	40,413,000	24,082,000	34,549,000
Operating income (loss)	(40,413,000)	(24,082,000)	(34,549,000)
Other income (loss), net	5,880,000	2,988,000	1,532,000
Interest income	710,000	248,000	97,000
Income (loss) from continuing operations before income taxes and noncontrolling interests	(33,823,000)	(20,846,000)	(32,920,000)
Income tax benefit (expense)	11,489,000	7,296,000	144,000
Income (loss) from continuing operations	(22,334,000)	(13,550,000)	(32,776,000)
Income (loss) from discontinued operations, including gain on sale, net of tax	193,877,000	79,399,000	(67,569,000)
Net income (loss)	171,543,000	65,849,000	(100,345,000)
Less: Net income (loss) attributable to the noncontrolling interests	(2,415,000)	(4,236,000)	(4,265,000)
Net income (loss) attributable to Actua Corporation	173,958,000	70,085,000	(96,080,000)
Amounts attributable to Actua Corporation:
Net income (loss) from continuing operations	(22,334,000)	(13,550,000)	(32,776,000)
Net income (loss) from discontinued operations	196,292,000	83,635,000	(63,304,000)
Net income (loss) attributable to Actua Corporation	$ 173,958,000	$ 70,085,000	$ (96,080,000)
Basic and diluted income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations (in dollars per share)	$ (0.72)	$ (0.37)	$ (0.89)
Income (loss) from discontinued operations (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
Net income (loss)	$ 171,543,000	$ 65,849,000	$ (100,345,000)
Other comprehensive income (loss)
Foreign currency translation adjustment	(99,000)	(59,000)	0
Comprehensive income (loss)	171,444,000	65,790,000	(100,345,000)
Less: Comprehensive income (loss) attributable to noncontrolling interests	(2,415,000)	(4,236,000)	(4,265,000)
Comprehensive income (loss) attributable to Actua Corporation	$ 173,859,000	$ 70,026,000	$ (96,080,000)