Consolidated Businesses - Pro Forma Information (Detail) - FolioDynamix, Ludwig, NuCivic, KMI - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Business Acquisition [Line Items]
Revenue	$ 34,100,000	$ 29,700,000	$ 98,800,000	$ 84,600,000
Net income (loss) from continuing operations attributable to Actua Corporation	$ (13,900,000)	$ (14,400,000)	$ (43,900,000)	$ (42,000,000)
Net income (loss) from continuing operations per diluted share attributable to Actua Corporation	$ (370,000.00)	$ (390,000.00)	$ (1,190,000.00)	$ (1,130,000.00)