Discontinued Operations - Loss from Discontinued Operations, net of tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Major classes of line items constituting pretax loss of discontinued operations
Total discontinued operations, net of income taxes	$ (1,018)	$ (8,649)	$ (9,003)	$ (24,692)
Discontinued operations attributable to Actua Corporation	(594)	(7,630)	(7,634)	(21,804)
GovDelivery | Disposed of by sale
Major classes of line items constituting pretax loss of discontinued operations
Revenue	31,315	39,298	93,011	110,617
Cost of revenue	(7,700)	(11,995)	(23,378)	(33,027)
Selling, general, administrative and other operating expenses	(24,152)	(35,724)	(77,429)	(101,459)
Other non-major income and expense items	(248)	(135)	(525)	(502)
Total discontinued operations before income taxes	(785)	(8,556)	(8,321)	(24,371)
Income tax benefit (expense)	(233)	(93)	(682)	(321)
Total discontinued operations, net of income taxes	(1,018)	(8,649)	(9,003)	(24,692)
Less: discontinued operations attributable to noncontrolling interests	(424)	(1,019)	(1,369)	(2,888)
Discontinued operations attributable to Actua Corporation	$ (594)	$ (7,630)	$ (7,634)	$ (21,804)