Ownership Interests in and Advances to Partner Companies and Funds - Narrative (Detail) - USD ($)	1 Months Ended					12 Months Ended
	Jul. 31, 2018	May 31, 2018	Feb. 28, 2018	Jan. 31, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment [Line Items]
Equity Method Investment, Other than Temporary Impairment						$ 12,600,000
Proceeds from sale of business						67,387,000	$ 16,604,000
Asset impairment charges							16,000,000
Adjusted carrying value of capital						64,489,000	108,089,000
Equity method investment assets exceed carrying value of investment						43,000,000
Intangible Assets [Member]
Investment [Line Items]
Equity method investment allocation of excess fair value						33,000,000
Goodwill [Member]
Investment [Line Items]
Equity method investment allocation of excess fair value						10,000,000
Flashtalking, Inc. [Member]
Investment [Line Items]
Cost Method Investments, Original Cost				$ 11,200,000
Cost-method Investments, Realized Gain (Loss)				3,900,000
Other Investments and Securities, at Cost						$ 11,000,000
Aventura [Member]
Investment [Line Items]
Proceeds from Sale of Equity Method Investments				600,000
Equity Method Investment, Realized Gain (Loss) on Disposal				$ 600,000
Asset impairment charges							200,000
Adjusted carrying value of capital							0	$ 6,200,000
Nexxt, Inc. [Member]
Investment [Line Items]
Repayments of Debt			$ 10,500,000
Gain on sale of business			9,500,000
Consideration received					$ 26,000,000
Proceeds from sale of business			$ 10,500,000		$ 15,500,000
Term of note receivable (in years)			3 years
Noncash or Part Noncash Divestiture, Interest Rate on Note Receivable			9.50%
Loans Receivable, Net			$ 10,500,000				0
Good Start Genetics Inc [Member]
Investment [Line Items]
Number of shares received, amount held in escrow (in shares)						45,989
Gain on sale of business						$ 1,100,000	4,300,000
Trading securities sold						492,340
Proceeds from sale of securities						$ 3,700,000
Adjusted carrying value of capital								17,200,000
Cask Data [Member]
Investment [Line Items]
Proceeds from Sale of Equity Method Investments		$ 11,500,000
Equity Method Investments, Amount in Holdbacks and Escrows		2,400,000
Equity Method Investment, Realized Gain (Loss) on Disposal		$ 4,200,000
Adjusted carrying value of capital							13,000,000	11,000,000
Media Math Inc [Member]
Investment [Line Items]
Proceeds from Sale of Equity Method Investments	$ 45,000,000
Equity Method Investments, Percent of Ownership Sold, Repurchase Option Within 180 Days	10.87%
Equity Method Investments, Sale Price, Repurchase Option Within 180 Days	$ 12,500,000
Equity Method Investment, Realized Gain (Loss) on Disposal	$ 45,000,000
Ownership percent sold	39.13%
Advantedge Healthcare Solutions Inc [Member]
Investment [Line Items]
Proceeds from Sale of Equity Method Investments	$ 10,000,000
Equity Method Investments, Sale of Investments, Additional Amount Receivable After Certain Valuation Thresholds	6,300,000
Equity Method Investment, Realized Gain (Loss) on Disposal	$ 5,500,000
Full Measure [Member]
Investment [Line Items]
Asset impairment charges							7,000,000
Adjusted carrying value of capital								8,600,000
Spongecell [Member]
Investment [Line Items]
Asset impairment charges							3,600,000
Adjusted carrying value of capital							18,600,000	$ 18,000,000
Pneuron [Member]
Investment [Line Items]
Asset impairment charges							$ 5,200,000
Flashtalking, Inc. [Member]
Investment [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners				10.00%