Discontinued Operations - Loss from Discontinued Operations, net of tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Major classes of line items constituting pretax loss of discontinued operations
Total discontinued operations, net of income taxes	$ 0	$ (2,079)	$ 0	$ (4,160)
Discontinued operations attributable to Actua Corporation	$ 0	(1,978)	$ 0	(3,949)
GovDelivery | Disposed of by sale
Major classes of line items constituting pretax loss of discontinued operations
Revenue		9,855		19,279
Cost of revenue		(3,552)		(7,022)
Selling, general, administrative and other operating expenses		(8,283)		(16,218)
Other non-major income and expense items		(90)		(169)
Total discontinued operations before income taxes		(2,070)		(4,130)
Income tax benefit (expense)		(9)		(30)
Total discontinued operations, net of income taxes		(2,079)		(4,160)
Less: discontinued operations attributable to noncontrolling interests		(101)		(211)
Discontinued operations attributable to Actua Corporation		$ (1,978)		$ (3,949)