Summary of Selected Information Related to Segments (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	$ (10,680)	$ (3,332)	$ (33,724)	$ 8,878
General and administrative
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	(9,787)	(5,023)	(25,713)	(15,532)
Interest income
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	157	34	371	104
Impairment related and other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	(256)		(1,019)	(33)
Corporate other income (loss)
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	70	6	1,017	(97)
Income tax benefit (expense)
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	2,131		2,931
Noncontrolling interest (income) loss
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	1,184	907	3,563	4,951
Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	$ (6,501)	$ (4,076)	$ (18,850)	$ (10,607)