Consolidated Businesses - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Business Acquisition [Line Items]
Impact of subsidiary equity transactions		$ 2,360	$ 4,453
Payments to acquire additional interest in subsidiaries	$ 5,400		5,400
Actua's share of consolidated business equity transactions			1,100
Additional Paid-In Capital
Business Acquisition [Line Items]
Impact of subsidiary equity transactions		$ 2,531	$ 4,314