Segment Information - Summary of Selected Information Related to Segments (Detail) - USD ($) $ 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
Segment Reporting Information [Line Items]
Revenue	$ 35,153	$ 34,140	$ 33,536	$ 30,592	$ 26,624	$ 20,762	$ 19,029	$ 18,422	$ 133,421	$ 84,837	$ 59,201
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
Assets	450,144				528,912				450,144	528,912	529,718
Capital Expenditures									(5,530)	(3,677)	(1,243)
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)
Impairment related and other	(39,978)	(424)	(490)	(346)	(95)	(256)	(836)	0	(41,238)	(1,187)	(4,292)
Corporate other income (loss)	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
Income tax benefit (expense)	(4)	(47)	(1)	(177)	10,556	1,870	599	(94)	(229)	12,931	17,571
Net loss attributable to the noncontrolling interest	1,593	$ 751	$ 761	$ 1,160	701	$ 1,184	$ 1,475	$ 904	4,265	4,264	7,018
Operating Segments
Segment Reporting Information [Line Items]
Revenue									133,421	84,837	59,201
Net income (loss) attributable to Actua Corporation									(68,538)	(20,445)	(20,346)
Assets	393,182				433,994				393,182	433,994	195,680
Capital Expenditures									(5,493)	(3,334)	(1,209)
Operating Segments | Vertical Cloud
Segment Reporting Information [Line Items]
Revenue									133,421	84,837	58,772
Net income (loss) attributable to Actua Corporation									(68,538)	(19,669)	(18,026)
Assets	376,565				417,877				376,565	417,877	179,801
Capital Expenditures									(5,493)	(3,334)	(1,183)
Operating Segments | Vertical Cloud Venture
Segment Reporting Information [Line Items]
Revenue									0	0	429
Net income (loss) attributable to Actua Corporation									0	(776)	(2,320)
Assets	16,617				16,117				16,617	16,117	15,879
Capital Expenditures									0	0	(26)
Reconciling Items, Dispositions
Segment Reporting Information [Line Items]
Revenue									0	0	0
Net income (loss) attributable to Actua Corporation									0	14,026	230,773
Assets	0				2,998				0	2,998	0
Capital Expenditures									0	0	0
Reconciling Items, Other
Segment Reporting Information [Line Items]
Revenue									0	0	0
Net income (loss) attributable to Actua Corporation									(27,542)	(17,137)	(1,368)
Assets	$ 56,962				$ 91,920				56,962	91,920	334,038
Capital Expenditures									(37)	(343)	(34)
Reconciling Items, Other | General and administrative
Segment Reporting Information [Line Items]
General and administrative									(33,521)	(35,082)	(19,132)
Reconciling Items, Other | Impairment related and other
Segment Reporting Information [Line Items]
Impairment related and other									85	(1,114)	(2,953)
Reconciling Items, Other | Corporate other income (loss)
Segment Reporting Information [Line Items]
Corporate other income (loss)									1,532	5,427	(4,102)
Reconciling Items, Other | Interest income
Segment Reporting Information [Line Items]
Interest income									97	434	171
Reconciling Items, Other | Income tax benefit (expense)
Segment Reporting Information [Line Items]
Income tax benefit (expense)									0	8,934	17,630
Reconciling Items, Other | Net loss attributable to the noncontrolling interest
Segment Reporting Information [Line Items]
Net loss attributable to the noncontrolling interest									$ 4,265	$ 4,264	$ 7,018