Actua Corporation's Stockholders' Equity (Details)		12 Months Ended
	Feb. 01, 2018 USD ($)	Dec. 31, 2017 USD ($) Vote shares	Dec. 31, 2016 shares
Subsequent Event [Line Items]
Number of votes per share | Vote		1
Dividends | $		$ 0
Preferred stock, shares authorized (in shares) | shares		10,000,000	10,000,000
Preferred stock, shares issued (in shares) | shares		0	0
Subsequent Event
Subsequent Event [Line Items]
Net proceeds | $	$ 476,000,000