Consolidated Businesses - Pro Forma Information (Detail) - FolioDynamix, Ludwig, NuCivic, KMI - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Business Acquisition [Line Items]
Revenue	$ 33,500	$ 28,200	$ 64,100	$ 54,800
Net income (loss) from continuing operations attributable to Actua Corporation	$ (15,300)	$ (15,800)	$ (30,000)	$ (27,600)
Net income (loss) from continuing operations per diluted share attributable to Actua Corporation	$ (0.41)	$ (0.42)	$ (0.81)	$ (0.74)