Ownership Interests in and Advances to Partner Companies and Funds - Narrative (Detail) - USD ($)	1 Months Ended							12 Months Ended
	May 31, 2019	Jan. 31, 2019	Jul. 31, 2018	May 31, 2018	Feb. 28, 2018	Jan. 31, 2018	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Equity Method Investment, Other than Temporary Impairment								$ 3,000,000	$ 12,600,000
Equity Securities, FV-NI, Unrealized Gain (Loss)								4,500,000
Proceeds from sale of business								104,302,000	67,387,000
Adjusted carrying value of capital								34,542,000	64,489,000
Equity method investment assets exceed carrying value of investment								24,300,000
Intangible Assets [Member]
Investment [Line Items]
Equity method investment allocation of excess fair value								19,000,000
Goodwill [Member]
Investment [Line Items]
Equity method investment allocation of excess fair value								5,300,000
Propeller [Member]
Investment [Line Items]
Proceeds from Sale of Equity Method Investments		$ 41,500,000
Equity Method Investments, Amount in Holdbacks and Escrows		700,000
Equity Method Investment, Realized Gain (Loss) on Disposal		$ 35,100,000
Adjusted carrying value of capital										$ 14,000,000
Flashtalking, Inc. [Member]
Investment [Line Items]
Cost-method Investments, Realized Gain (Loss)						$ 3,900,000
Other Investments and Securities, at Cost								$ 11,000,000	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
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%
Financing Receivable, after Allowance for Credit Loss					$ 10,500,000					0
Good Start Genetics Inc [Member]
Investment [Line Items]
Gain on sale of business									$ 1,100,000	4,300,000
Trading securities sold								27,264	492,340
Proceeds from sale of securities								$ 700,000	$ 3,700,000
Cask Data [Member]
Investment [Line Items]
Proceeds from Sale of Equity Method Investments				$ 11,500,000				3,800,000
Equity Method Investment, Realized Gain (Loss) on Disposal				$ 4,200,000
Adjusted carrying value of capital										$ 13,000,000
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
Transactis [Member]
Investment [Line Items]
Proceeds from Sale of Equity Method Investments	$ 57,500,000
Equity Method Investment, Realized Gain (Loss) on Disposal								$ 50,700,000
Flashtalking, Inc. [Member]
Investment [Line Items]
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners						10.00%