Segment Information - Summary of Selected Information Related to Segments (Detail) - USD ($) $ 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
Segment Reporting Information [Line Items]
Revenue	$ 29,324	$ 27,937	$ 26,854	$ 25,186	$ 25,475	$ 24,917	$ 25,307	$ 22,733	$ 109,301	$ 98,432	$ 57,129
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)
Assets	453,718				450,144				453,718	450,144	528,341
Capital expenditures									(2,601)	(2,237)	(1,188)
Segment Reporting Information, Profit (Loss) [Abstract]
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)
Impairment related and other	(375)	(45)	(117)	(231)	(39,787)	(285)	(430)	(267)	(768)	(40,769)	(1,114)
Other income (loss), net (refer to Note 13)	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
Income tax benefit (expense)	13,501	(93)	(72)	(126)	(89)	(29)	0	(175)	13,210	(293)	10,528
Net 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)
Revenue	29,324	$ 27,937	$ 26,854	$ 25,186	25,475	$ 24,917	$ 25,307	$ 22,733	109,301	98,432	57,129
Operating Segments
Segment Reporting Information [Line Items]
Revenue									109,301	98,432	57,129
Net income (loss) attributable to Actua Corporation									(23,339)	(58,891)	(15,043)
Assets	375,772				365,582				375,772	365,582	406,943
Capital expenditures									(2,589)	(2,200)	(845)
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation									(23,339)	(58,891)	(15,043)
Revenue									109,301	98,432	57,129
Operating Segments | Vertical cloud
Segment Reporting Information [Line Items]
Revenue									109,301	98,432	57,129
Net income (loss) attributable to Actua Corporation									(23,339)	(58,891)	(14,267)
Assets	358,522				347,436				358,522	347,436	389,271
Capital expenditures									(2,589)	(2,200)	(845)
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation									(23,339)	(58,891)	(14,267)
Revenue									109,301	98,432	57,129
Operating Segments | Vertical cloud (venture)
Segment Reporting Information [Line Items]
Revenue									0	0	0
Net income (loss) attributable to Actua Corporation									0	0	(776)
Assets	17,250				18,146				17,250	18,146	17,672
Capital expenditures									0	0	0
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation									0	0	(776)
Revenue									0	0	0
Dispositions
Segment Reporting Information [Line Items]
Revenue									0	0	0
Net income (loss) attributable to Actua Corporation									95,295	(8,520)	11,316
Assets	0				29,129				0	29,129	31,408
Capital expenditures									0	0	0
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation									95,295	(8,520)	11,316
Revenue									0	0	0
Other
Segment Reporting Information [Line Items]
Revenue									0	0	0
Net income (loss) attributable to Actua Corporation									(1,871)	(28,669)	(19,829)
Assets	$ 77,946				$ 55,433				77,946	55,433	89,990
Capital expenditures									(12)	(37)	(343)
Segment Reporting Information, Profit (Loss) [Abstract]
General and administrative									(23,781)	(33,996)	(35,082)
Impairment related and other									(42)	85	(1,114)
Other income (loss), net (refer to Note 13)									2,988	1,532	5,427
Interest income									249	97	434
Income tax benefit (expense)									15,757	0	6,496
Net loss attributable to the noncontrolling interests									2,958	3,613	4,010
Net income (loss) attributable to Actua Corporation									(1,871)	(28,669)	(19,829)
Revenue									$ 0	$ 0	$ 0