Net Loss per Share - Calculations of Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic and Diluted:
Net loss attributable to Actua Corporation	$ (12,741)	$ (15,282)	$ (27,072)	$ (30,047)
Net loss attributable to Actua Corporation per share (in dollars per share)	$ (0.35)	$ (0.41)	$ (0.73)	$ (0.81)
Shares used in computation of basic and diluted loss per share (in shares)	36,760	37,123	37,027	36,983