Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Summary of Selected Information Related to Segments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Revenue	$ 26,624us-gaap_Revenues	$ 20,762us-gaap_Revenues	$ 19,029us-gaap_Revenues	$ 18,422us-gaap_Revenues	$ 17,680us-gaap_Revenues	$ 16,071us-gaap_Revenues	$ 13,476us-gaap_Revenues	$ 11,974us-gaap_Revenues	$ 84,837us-gaap_Revenues	$ 59,201us-gaap_Revenues	$ 26,640us-gaap_Revenues
Net income (loss) attributable to Actua Corporation	10,168us-gaap_NetIncomeLoss	(10,680)us-gaap_NetIncomeLoss	(12,894)us-gaap_NetIncomeLoss	(10,150)us-gaap_NetIncomeLoss	200,181us-gaap_NetIncomeLoss	(3,332)us-gaap_NetIncomeLoss	(6,858)us-gaap_NetIncomeLoss	19,068us-gaap_NetIncomeLoss	(23,556)us-gaap_NetIncomeLoss	209,059us-gaap_NetIncomeLoss	22,989us-gaap_NetIncomeLoss
Assets	528,341us-gaap_Assets				529,718us-gaap_Assets				528,341us-gaap_Assets	529,718us-gaap_Assets	447,059us-gaap_Assets
Capital expenditures, net									(3,677)	(1,243)	(6,990)
Operating Segments
Segment Reporting Information [Line Items]
Revenue									84,837	59,201	26,640
Net income (loss) attributable to Actua Corporation									(20,029)	(20,346)	(21,079)
Assets	433,423				195,680				433,423	195,680	188,120
Capital expenditures, net									(3,334)	(1,209)	(5,487)
Vertical Cloud \| Operating Segments
Segment Reporting Information [Line Items]
Revenue									84,837	58,772	26,026
Net income (loss) attributable to Actua Corporation									(19,253)	(18,026)	(18,244)
Assets	417,306				179,801				417,306	179,801	173,300
Capital expenditures, net									(3,334)	(1,183)	(5,487)
Vertical Cloud Venture \| Operating Segments
Segment Reporting Information [Line Items]
Revenue										429	614
Net income (loss) attributable to Actua Corporation									(776)	(2,320)	(2,835)
Assets	16,117				15,879				16,117	15,879	14,820
Capital expenditures, net										(26)
Dispositions \| Reconciling Items
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									14,026	230,773	10,440
Assets	2,998								2,998		242,938
Other \| Reconciling Items
Segment Reporting Information [Line Items]
Net income (loss) attributable to Actua Corporation									(17,553)	(1,368)	33,628
Assets	91,920				334,038				91,920	334,038	16,001
Capital expenditures, net									$ (343)	$ (34)	$ (1,503)