Consolidated Businesses - Pro Forma Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ErgoAdvocate, WellNet, and E3 Solutions
Business Acquisition [Line Items]
Revenue	$ 113,426	$ 102,726
Net income (loss) from continuing operations attributable to Actua Corporation	$ (23,244)	$ (88,696)
Net income (loss) from continuing operations per diluted share attributable to Actua Corporation (in dollars per share)	$ (0.63)	$ (2.39)
WellNet
Business Acquisition [Line Items]
Revenue		$ 98,645	$ 57,309
Net income (loss) from continuing operations attributable to Actua Corporation		$ (87,773)	$ (32,434)
Net income (loss) from continuing operations per diluted share attributable to Actua Corporation (in dollars per share)		$ (2.37)	$ (0.87)