CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenue	$ 0	$ 0	$ 0	$ 0
Operating expenses
Cost of revenue	0	0	0	0
Sales and marketing	0	0	0	0
General and administrative	6,186	5,313	19,607	18,280
Research and development	0	0	0	0
Amortization of intangible assets	0	0	0	0
Impairment related and other	0	(3)	0	42
Total operating expenses	6,186	5,310	19,607	18,322
Operating income (loss)	(6,186)	(5,310)	(19,607)	(18,322)
Other income (loss), net	192	2,842	3,846	2,888
Interest income	107	24	381	92
Income (loss) before income taxes and noncontrolling interests	(5,887)	(2,444)	(15,380)	(15,342)
Income tax benefit (expense)	0	0	0	0
Income (loss) from continuing operations	(5,887)	(2,444)	(15,380)	(15,342)
Income (loss) from discontinued operations, net of tax	(1,018)	(8,649)	(9,003)	(24,692)
Net income (loss)	(6,905)	(11,093)	(24,383)	(40,034)
Less: Net income (loss) attributable to the noncontrolling interests	(424)	(1,019)	(1,369)	(2,888)
Net income (loss) attributable to Actua Corporation	(6,481)	(10,074)	(23,014)	(37,146)
Amounts attributable to Actua Corporation:
Net income (loss) from continuing operations	(5,887)	(2,444)	(15,380)	(15,342)
Net income (loss) from discontinued operations	(594)	(7,630)	(7,634)	(21,804)
Net income (loss) attributable to Actua Corporation	$ (6,481)	$ (10,074)	$ (23,014)	$ (37,146)
Basic and diluted income (loss) per share attributable to Actua Corporation:
Income (loss) from continuing operations (in dollars per share)	$ (0.19)	$ (0.06)	$ (0.49)	$ (0.42)
Income (loss) from discontinued operations (in dollars per share)	(0.02)	(0.21)	(0.24)	(0.59)
Net income (loss) (in dollars per share)	$ (0.21)	$ (0.27)	$ (0.73)	$ (1.01)
Shares used in computation of basic and diluted income (loss) per share (in shares)	30,641	36,776	31,335	36,943
Net income (loss)	$ (6,905)	$ (11,093)	$ (24,383)	$ (40,034)
Other comprehensive income (loss):
Foreign currency translation adjustment	(69)	71	(115)	(41)
Comprehensive income (loss)	(6,974)	(11,022)	(24,498)	(40,075)
Less: Comprehensive income (loss) attributable to the noncontrolling interests	(424)	(1,019)	(1,369)	(2,888)
Comprehensive income (loss) attributable to Actua Corporation	$ (6,550)	$ (10,003)	$ (23,129)	$ (37,187)