Selected Quarterly Financial Information (Unaudited) (Details) - 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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 29,324	$ 27,937	$ 26,854	$ 25,186	$ 25,475	$ 24,917	$ 25,307	$ 22,733	$ 109,301	$ 98,432	$ 57,129
Operating expenses
Cost of revenue	7,735	7,441	7,288	6,722	6,887	6,663	7,011	6,859	29,186	27,420	16,299
Sales and marketing	9,946	10,281	10,461	10,119	8,921	9,551	8,922	8,438	40,807	35,832	28,556
General and administrative	12,010	11,228	11,372	12,326	13,627	12,542	14,394	13,953	46,936	54,516	47,031
Research and development	5,536	5,411	5,388	5,307	4,619	4,973	4,940	4,464	21,642	18,996	8,023
Amortization of intangibles assets	3,652	3,524	3,626	3,582	3,303	3,266	3,293	3,587	14,384	13,449	8,950
Impairment related and other	375	45	117	231	39,787	285	430	267	768	40,769	1,114
Total operating expenses	39,254	37,930	38,252	38,287	77,144	37,280	38,990	37,568	153,723	190,982	109,973
Operating income (loss)	(9,930)	(9,993)	(11,398)	(13,101)	(51,669)	(12,363)	(13,683)	(14,835)	(44,422)	(92,550)	(52,844)
Other income (loss), net	131	2,831	(15)	(90)	204	96	(52)	1,493	2,857	1,741	5,359
Interest income	165	31	39	48	43	30	35	18	283	126	455
Interest expense	(3)	(27)	(33)	(33)	(93)	(36)	(31)	(37)	(96)	(197)	(1,604)
Income (loss) from continuing operations before income taxes, equity loss and noncontrolling interests	(9,637)	(7,158)	(11,407)	(13,176)	(51,515)	(12,273)	(13,731)	(13,361)	(41,378)	(90,880)	(48,634)
Income tax benefit (expense)	13,501	(93)	(72)	(126)	(89)	(29)	0	(175)	13,210	(293)	10,528
Income (loss) from continuing operations	3,864	(7,251)	(11,479)	(13,302)	(51,604)	(12,302)	(13,731)	(13,536)	(28,168)	(91,173)	(38,882)
Income (loss) from discontinued operations, including gain on sale, net of tax	102,019	(3,842)	(2,079)	(2,081)	(2,140)	(2,331)	(2,312)	(2,389)	94,017	(9,172)	11,062
Net income (loss)	105,883	(11,093)	(13,558)	(15,383)	(53,744)	(14,633)	(16,043)	(15,925)	65,849	(100,345)	(27,820)
Less: Net income (loss) attributable to the noncontrolling interests	(1,348)	(1,019)	(817)	(1,052)	(1,593)	(751)	(761)	(1,160)	(4,236)	(4,265)	(4,264)
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)
Amounts attributable to Actua Corporation:
Net 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
Basic income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations (in dollars per share)	$ 0.12	$ (0.17)	$ (0.30)	$ (0.33)	$ (1.35)	$ (0.31)	$ (0.35)	$ (0.34)
Income (loss) from discontinued operations (in dollars per share)	2.87	(0.10)	(0.05)	(0.05)	(0.05)	(0.06)	(0.06)	(0.06)
Net income (loss) (in dollars per share)	$ 2.99	$ (0.27)	$ (0.35)	$ (0.38)	$ (1.40)	$ (0.37)	$ (0.41)	$ (0.40)
Shares used in computation of basic income (loss) per share (in shares)	35,865	36,776	36,760	37,293	37,190	37,146	37,123	36,842
Diluted income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations (in dollars per share)	$ 0.12	$ (0.17)	$ (0.30)	$ (0.33)	$ (1.35)	$ (0.31)	$ (0.35)	$ (0.34)
Income (loss) from discontinued operations (in dollars per share)	2.82	(0.10)	(0.05)	(0.05)	(0.05)	(0.06)	(0.06)	(0.06)
Net income (loss) (in dollars per share)	$ 2.94	$ (0.27)	$ (0.35)	$ (0.38)	$ (1.40)	$ (0.37)	$ (0.41)	$ (0.40)
Shares used in computation of diluted income (loss) per share (in shares)	36,421	36,776	36,760	37,293	37,190	37,146	37,123	36,842