Ownership Interests in and Advances to Partner Companies and Funds - Narrative (Detail) - USD ($) $ in Thousands	1 Months Ended							3 Months Ended		6 Months Ended	9 Months Ended
	Aug. 31, 2017	Apr. 30, 2017	Mar. 31, 2017	Jan. 31, 2017	Jul. 31, 2016	Apr. 30, 2016	Jul. 31, 2015	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Mar. 03, 2017	Dec. 31, 2016
Investment [Line Items]
Proceeds from sale of business											$ 16,604	$ 73,854
Equity method investments											114,313			$ 154,666
Good Start Genetics, Inc.
Investment [Line Items]
Number of shares received (in shares)	414,237
Amount of consideration received	$ 4,300
Number of shares received, amount held in escrow (in shares)	137,125
Amount of consideration received, amount held in escrow	$ 1,400
Gain on sale of business											4,300
Pneuron
Investment [Line Items]
Asset impairment charges								$ 2,200	$ 2,700
Equity method investments											0
Putney Inc
Investment [Line Items]
Proceeds from sale of business		$ 700				$ 58,600
Gain on sale of business										$ 700
Beyond.com, Inc
Investment [Line Items]
Amount of consideration received			$ 10,500
Consideration received per transaction			26,000
Proceeds from sale of business			$ 15,500
Term of note receivable (in years)			3 years
Interest rate on note receivable													9.50%
Gain on sale of business											100
Quantia, Inc.
Investment [Line Items]
Proceeds from sale of business				$ 600	$ 600		$ 7,800
Gain on sale of business											$ 600
Spongecell Inc.
Investment [Line Items]
Asset impairment charges								$ 3,600