Ownership Interests in and Advances to Partner Companies and Funds - Narrative (Detail) - USD ($)	1 Months Ended				3 Months Ended
	Feb. 28, 2018	Jan. 31, 2018	Aug. 31, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Mar. 03, 2017
Investment [Line Items]
Proceeds from sale of business					$ 3,257,000	$ 15,753,000
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		$ 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)					4,000,000
Invitae Corporation
Investment [Line Items]
Trading securities sold	414,237
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, equity							$ 3,800,000
Trading securities, realized gain (loss)					1,200,000
Aventura
Investment [Line Items]
Proceeds from sale of equity method investments		$ 600,000
Gain (loss) on sale of equity investments					$ 600,000
Beyond.com, Inc
Investment [Line Items]
Repayments of debt	$ 10,500,000