Selected Quarterly Consolidated Financial Information (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 26,624		$ 20,762		$ 19,029		$ 18,422		$ 17,680		$ 16,071		$ 13,476		$ 11,974		$ 84,837	$ 59,201	$ 26,640
Operating expenses
Cost of Revenue	8,091		5,911		5,519		4,899		4,935		4,501		4,123		4,198		24,420	17,757	9,459
Sales and marketing	10,876		10,718		9,585		8,531		8,335		7,571		6,541		5,682		39,710	28,129	12,355
General and administrative	15,009		13,798		12,915		10,009		7,085		7,852		7,220		8,803		51,731	30,960	28,408
Research and development	4,648		3,960		3,547		3,253		2,306		2,149		2,331		2,246		15,408	9,032	8,807
Amortization of intangibles assets	3,569		2,369		2,293		2,301		2,259		2,176		1,544		2,491		10,532	8,470	4,837
Impairment related and other	95		256		836				3,525		470		127		170		1,092		1,130
Total operating expenses	42,288		37,012		34,695		28,993		28,445		24,719		21,886		23,590		142,988	98,640	64,996
Operating income (loss)	(15,664)		(16,250)		(15,666)		(10,571)		(10,765)		(8,648)		(8,410)		(11,616)		(58,151)	(39,439)	(38,356)
Other income (loss), net	4,280		83		637		300		(4,032)		(68)		(46)		(64)		5,300	(4,210)	57,820
Interest income	71		160		146		86		77		63		56		31		463	227	439
Interest expense	(13)		(9)		(1,080)		(511)		(408)		(370)		(385)		(321)		(1,613)	(1,484)	(25)
Income (loss) before income taxes, equity loss and noncontrolling interest	(11,326)		(16,016)		(15,963)		(10,696)		(15,128)		(9,023)		(8,785)		(11,970)		(54,001)	(44,906)	19,878
Income tax (expense) benefit	10,556		1,870		599		(94)		17,601		99		(56)		(73)		12,931	17,571	(1,336)
Equity loss			(144)		(320)		(312)		(1,044)		(295)		(923)		(701)		(776)	(2,963)	(8,672)
Income (loss) from continuing operations	(770)		(14,290)		(15,684)		(11,102)		1,429		(9,219)		(9,764)		(12,744)		(41,846)	(30,298)	11,098
Income (loss) from discontinued operations	10,237		2,426		1,315		48		196,685		4,980		2,448		28,226		14,026	232,339	12,483
Net income (loss)	9,467		(11,864)		(14,369)		(11,054)		198,114		(4,239)		(7,316)		15,482		(27,820)	202,041	23,581
Less: Net income (loss) attributable to the noncontrolling interest	(701)		(1,184)		(1,475)		(904)		(2,067)		(907)		(458)		(3,586)		(4,264)	(7,018)	592
Net income (loss) attributable to Actua Corporation	10,168		(10,680)		(12,894)		(10,150)		200,181		(3,332)		(6,858)		19,068		(23,556)	209,059	22,989
Amounts attributable to Actua Corporation:
Net income (loss) from continuing operations	(69)		(13,106)		(14,209)		(10,198)		2,804		(8,022)		(9,025)		(11,426)		(37,582)	(25,669)	12,703
Net income (loss) from discontinued operations	10,237		2,426		1,315		48		197,377		4,690		2,167		30,494		14,026	234,728	10,286
Net income (loss) attributable to Actua Corporation	$ 10,168		$ (10,680)		$ (12,894)		$ (10,150)		$ 200,181		$ (3,332)		$ (6,858)		$ 19,068		$ (23,556)	$ 209,059	$ 22,989
Basic income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations	$ 0.00	[1]	$ (0.35)	[1]	$ (0.38)	[1]	$ (0.27)	[1]	$ 0.08	[1]	$ (0.22)	[1]	$ (0.25)	[1]	$ (0.31)	[1]	$ (1.01)	$ (0.70)	$ 0.35
Income from discontinued operations	$ 0.28	[1]	$ 0.06	[1]	$ 0.03	[1]	$ 0.00	[1]	$ 5.38	[1]	$ 0.13	[1]	$ 0.06	[1]	$ 0.83	[1]	$ 0.38	$ 6.42	$ 0.29
Net income (loss)	$ 0.28	[1]	$ (0.29)	[1]	$ (0.35)	[1]	$ (0.27)	[1]	$ 5.46	[1]	$ (0.09)	[1]	$ (0.19)	[1]	$ 0.52	[1]	$ (0.63)	$ 5.72	$ 0.64
Shares used in computation of basic income (loss) per share	36,780	[1]	37,335	[1]	37,313	[1]	37,096	[1]	36,664	[1]	36,303	[1]	36,468	[1]	36,713	[1]	37,130	36,536	35,890
Diluted income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations	$ 0.00	[1]	$ (0.35)	[1]	$ (0.38)	[1]	$ (0.27)	[1]	$ 0.08	[1]	$ (0.22)	[1]	$ (0.25)	[1]	$ (0.31)	[1]	$ (1.01)	$ (0.70)	$ 0.35
Income (loss) from discontinued operations	$ 0.28	[1]	$ 0.06	[1]	$ 0.03	[1]	$ 0.00	[1]	$ 5.10	[1]	$ 0.13	[1]	$ 0.06	[1]	$ 0.83	[1]	$ 0.38	$ 6.42	$ 0.28
Net income (loss)	$ 0.28	[1]	$ (0.29)	[1]	$ (0.35)	[1]	$ (0.27)	[1]	$ 5.18	[1]	$ (0.09)	[1]	$ (0.19)	[1]	$ 0.52	[1]	$ (0.63)	$ 5.72	$ 0.63
Shares used in computation of diluted income (loss) per share	36,780	[1]	37,335	[1]	37,313	[1]	37,096	[1]	38,680	[1]	36,303	[1]	36,468	[1]	36,713	[1]	37,130	36,536	36,543

[1] The sum of quarterly income (loss) per share differs from the full year amount due to changes in the number of shares outstanding during the year.