Summary of Selected Information Related to Segments (Parenthetical) (Detail) (USD $) In Thousands, 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
Segment Reporting Information [Line Items]
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
Impairment related and other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									(1,114)	(2,953)	(865)
Corporate other income (loss)
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									5,427	(4,102)	57,824
Income Tax Benefit
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									8,934	17,630	1
Equity loss
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation											(1)
Noncontrolling interest (income) loss
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									4,264	7,018	(592)
Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									(17,553)	(1,368)	33,628
General and administrative
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									(35,498)	(19,132)	(23,113)
Interest income
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									$ 434	$ 171	$ 374