Summary of Selected Information Related to Segments (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	$ (14,765)	$ (10,150)
General and administrative
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	(9,291)	(6,551)
Interest income
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	18	81
Impairment related and other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	(95)
Corporate other income (loss)
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	1,390	294
Net loss attributable to the noncontrolling interest
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	1,160	904
Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation	$ (6,818)	$ (5,272)