Segment Information - Summary of Selected Information Related to Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenue	$ 34,140	$ 20,762	$ 98,268	$ 58,213
Net income (loss) attributable to Actua Corporation	(13,882)	(10,680)	(43,929)	(33,724)
Income tax benefit (expense)	47	(1,870)	225	(2,375)
Net loss attributable to the noncontrolling interest	751	1,184	2,672	3,563
Assets	504,366		504,366		$ 528,912
Operating Segments
Segment Reporting Information [Line Items]
Revenue	34,140	20,762	98,268	58,213
Net income (loss) attributable to Actua Corporation	(6,420)	(6,605)	(21,511)	(18,663)
Assets	446,309		446,309		433,994
Reconciling Items, Dispositions
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Net income (loss) attributable to Actua Corporation	0	2,426	0	3,789
Assets	0		0		2,998
Reconciling Items, Other
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Net income (loss) attributable to Actua Corporation	(7,462)	(6,501)	(22,418)	(18,850)
Assets	58,057		58,057		91,920
Vertical Cloud | Operating Segments
Segment Reporting Information [Line Items]
Revenue	34,140	20,762	98,268	58,213
Net income (loss) attributable to Actua Corporation	(6,420)	(6,461)	(21,511)	(17,887)
Assets	429,692		429,692		417,877
Vertical Cloud (Venture) | Operating Segments
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Net income (loss) attributable to Actua Corporation	0	(144)	0	(776)
Assets	16,617		16,617		$ 16,117
General and administrative | Reconciling Items, Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	(8,148)	(9,787)	(26,255)	(25,713)
Impairment related and other | Reconciling Items, Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	(89)	(256)	(290)	(1,019)
Corporate Other Income (Loss) | Reconciling Items, Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	(1)	70	1,388	1,017
Interest income | Reconciling Items, Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	25	157	67	371
Income Tax Benefit (Expense) | Reconciling Items, Other
Segment Reporting Information [Line Items]
Income tax benefit (expense)	0	2,131	0	2,931
Net loss attributable to the noncontrolling interest | Reconciling Items, Other
Segment Reporting Information [Line Items]
Net loss attributable to the noncontrolling interest	$ 751	$ 1,184	$ 2,672	$ 3,563