Ownership Interests in and Advances to Partner Companies and Funds - Narrative (Detail) - USD ($)	1 Months Ended						3 Months Ended		9 Months Ended
	Jul. 26, 2018	May 31, 2018	Feb. 28, 2018	Jan. 31, 2018	Aug. 31, 2017	Mar. 31, 2017	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Mar. 03, 2017
Investment [Line Items]
Equity method investments							$ 71,688,000		$ 71,688,000			$ 108,089,000
Proceeds from sale of equity method investments	$ 55,000,000
Proceeds from sale of business									69,912,000	$ 16,604,000
Beyond.com, Inc
Investment [Line Items]
Repayments of debt			$ 10,500,000
CloudMine
Investment [Line Items]
Other than temporary impairment							4,800,000		4,800,000
Equity method investments											$ 10,500,000
Cask Data
Investment [Line Items]
Equity method investments								$ 13,300,000
Proceeds from sale of equity method investments		$ 11,500,000
Realized gain (loss) on disposal							4,200,000		4,200,000
Amount in holdbacks and escrows		$ 2,400,000
MediaMath, Inc.
Investment [Line Items]
Percent of ownership sold	39.13%
Proceeds from sale of equity method investments	$ 45,000,000
Percent of ownership sold, repurchase option within 180 days	10.87%
Repurchase option within 180 days	$ 12,500,000
Realized gain (loss) on disposal									45,000,000
AdvantEdge Healthcare Solutions, Inc.
Investment [Line Items]
Proceeds from sale of equity method investments	10,000,000
Realized gain (loss) on disposal							5,500,000		5,500,000
Additional amount receivable after certain valuation thresholds	$ 6,300,000
Apprenda
Investment [Line Items]
Other than temporary impairment									6,600,000
Equity method investments							$ 0		0
Beyond.com, Inc
Investment [Line Items]
Consideration received per transaction						$ 26,000,000
Proceeds from sale of business			$ 10,500,000			$ 15,500,000
Term of note receivable (in years)						3 years
Amount of consideration received						$ 10,500,000
Interest rate on note receivable													9.50%
Loans receivable, net						$ 10,500,000						0
Gain on sale of business									9,500,000
Flashtalking
Investment [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners				10.00%
Cost method investments, original cost				$ 11,200,000
Cost-method investments, realized gain (loss)									3,900,000
Invitae Corporation
Investment [Line Items]
Trading securities sold			414,237				78,103
Proceeds from sale of trading securities held-for-investment			$ 2,600,000					$ 2,600,000
Number of shares received (in shares)					414,237
Number of shares received, amount held in escrow (in shares)					124,092
Trading securities, realized gain (loss)							$ (1,100,000)		(1,100,000)
Trading securities, equity							1,300,000		1,300,000			$ 3,800,000
Unrealized holding gain (loss)							$ 200,000		(1,000,000)
Aventura
Investment [Line Items]
Proceeds from sale of equity method investments				$ 600,000
Realized gain (loss) on disposal									$ 600,000