Equity-Based Compensation - Total aggregate fair value of restricted stock and shares surrendered by Actua's employees (Detail) - Restricted stock - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Restricted stock vested in period, fair value	$ 106	$ 1,145	$ 8,548	$ 168	$ 776	$ 5,243	$ 9,799	$ 6,187
Shares paid for tax withholding for share based payments (in shares)	0	736	211,560	2,727	736	191,898	212,296	195,361