Selected Quarterly Financial Information (Unaudited) (Details) - 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
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 35,153	$ 34,140	$ 33,536	$ 30,592	$ 26,624	$ 20,762	$ 19,029	$ 18,422	$ 133,421	$ 84,837	$ 59,201
Operating expenses
Cost of revenue	10,087	9,627	9,783	9,732	8,091	5,911	5,519	4,899	39,229	24,420	17,757
Sales and marketing	12,070	12,945	12,163	11,236	10,876	10,718	9,585	8,531	48,414	39,710	28,129
General and administrative	16,015	14,440	15,966	15,436	15,009	13,798	12,915	10,009	61,857	51,731	30,960
Research and development	7,090	7,612	7,373	7,093	4,648	3,960	3,547	3,253	29,168	15,408	9,032
Amortization of intangibles assets	3,801	3,728	3,706	4,016	3,569	2,369	2,293	2,301	15,251	10,532	8,470
Impairment related and other	39,978	424	490	346	95	256	836	0	41,238	1,187	4,292
Total operating expenses	89,041	48,776	49,481	47,859	42,288	37,012	34,695	28,993	235,157	142,988	98,640
Operating income (loss)	(53,888)	(14,636)	(15,945)	(17,267)	(15,664)	(16,250)	(15,666)	(10,571)	(101,736)	(58,151)	(39,439)
Other income (loss), net	204	52	(101)	1,537	4,280	83	637	300	1,692	5,300	(4,210)
Interest income	43	34	35	19	71	160	146	86	131	463	227
Interest expense	(99)	(36)	(31)	(37)	(13)	(9)	(1,080)	(511)	(203)	(1,613)	(1,484)
Income (loss) before income taxes, equity loss and noncontrolling interest	(53,740)	(14,586)	(16,042)	(15,748)	(11,326)	(16,016)	(15,963)	(10,696)	(100,116)	(54,001)	(44,906)
Income tax (expense) benefit	(4)	(47)	(1)	(177)	10,556	1,870	599	(94)	(229)	12,931	17,571
Equity income (loss)	0	0	0	0	0	(144)	(320)	(312)	0	(776)	(2,963)
Income (loss) from continuing operations	(53,744)	(14,633)	(16,043)	(15,925)	(770)	(14,290)	(15,684)	(11,102)	(100,345)	(41,846)	(30,298)
Income (loss) from discontinued operations, including gain on sale, net of tax	0	0	0	0	10,237	2,426	1,315	48	0	14,026	232,339
Net income (loss)	(53,744)	(14,633)	(16,043)	(15,925)	9,467	(11,864)	(14,369)	(11,054)	(100,345)	(27,820)	202,041
Less: Net income (loss) attributable to the noncontrolling interest	(1,593)	(751)	(761)	(1,160)	(701)	(1,184)	(1,475)	(904)	(4,265)	(4,264)	(7,018)
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
Amounts attributable to Actua Corporation:
Net 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)
Net income (loss) from discontinued operations	$ 0	$ 0	$ 0	$ 0	$ 10,237	$ 2,426	$ 1,315	$ 48	$ 0	$ 14,026	$ 234,728
Basic and diluted income (loss) per share attributable to Actua Corporation(1):
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