Segment Information - Summary of Selected Information Related to Segments (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue	$ 31,189	$ 26,854	$ 61,696	$ 52,040
Net income (loss) attributable to Actua Corporation	(6,863)	(12,741)	(16,533)	(27,072)
Segment Reporting Information, Profit (Loss) [Abstract]
General and administrative	11,786	11,372	24,718	23,698
Impairment related and other expense	310	117	670	348
Other income (loss), net (refer to Note 11)	2,558	(15)	3,425	(105)
Interest income	142	39	297	87
Net (income) loss attributable to the noncontrolling interest	463	817	945	1,869
Net income (loss) attributable to Actua Corporation	(6,863)	(12,741)	(16,533)	(27,072)
Assets	414,639		414,639		$ 453,718
Operating Segments
Segment Reporting Information [Line Items]
Revenue	31,189	26,854	61,696	52,040
Net income (loss) attributable to Actua Corporation	(4,004)	(5,647)	(8,108)	(12,087)
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation	(4,004)	(5,647)	(8,108)	(12,087)
Assets	367,704		367,704		375,772
Operating Segments | Vertical Cloud
Segment Reporting Information [Line Items]
Revenue	31,189	26,854	61,696	52,040
Net income (loss) attributable to Actua Corporation	(4,004)	(5,647)	(8,108)	(12,087)
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation	(4,004)	(5,647)	(8,108)	(12,087)
Assets	350,231		350,231		358,522
Operating Segments | Vertical Cloud (Venture)
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Net income (loss) attributable to Actua Corporation	0	0	0	0
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation	0	0	0	0
Assets	17,473		17,473		17,250
Dispositions
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Net income (loss) attributable to Actua Corporation	0	(1,978)	0	(3,949)
Segment Reporting Information, Profit (Loss) [Abstract]
Net income (loss) attributable to Actua Corporation	0	(1,978)	0	(3,949)
Assets	0		0		0
Other
Segment Reporting Information [Line Items]
Revenue	0	0	0	0
Net income (loss) attributable to Actua Corporation	(2,859)	(5,116)	(8,425)	(11,036)	(8,425)
Segment Reporting Information, Profit (Loss) [Abstract]
General and administrative	(6,147)	(5,818)		(12,763)	13,298
Impairment related and other expense		(45)	0	(45)	0
Other income (loss), net (refer to Note 11)	2,697	0		46	3,654
Interest income	128	31		68	274
Net (income) loss attributable to the noncontrolling interest	463	716		(1,658)	945
Net income (loss) attributable to Actua Corporation	(2,859)	$ (5,116)	(8,425)	$ (11,036)	(8,425)
Assets	$ 46,935		$ 46,935		$ 77,946