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Ownership Interests in and Advances to Partner Companies and Funds - Narrative (Detail) - USD ($)	1 Months Ended									5 Months Ended	12 Months Ended
	Feb. 28, 2018	Aug. 31, 2017	Mar. 31, 2017	Jul. 31, 2016	Jun. 30, 2016	Apr. 30, 2016	Jan. 31, 2016	Jul. 31, 2015	Apr. 30, 2015	Jul. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment [Line Items]
Gain on sale of business												$ 424,000	$ 2,914,000
Proceeds from sale of business											$ 16,604,000	73,965,000	25,058,000
Asset impairment charges											216,000	2,360,000
Adjusted carrying value of capital											108,089,000	154,666,000
Equity method investment assets exceed carrying value of investment											77,500,000
Intangible Assets [Member]
Investment [Line Items]
Equity method investment allocation of excess fair value											60,900,000
Goodwill [Member]
Investment [Line Items]
Equity method investment allocation of excess fair value											16,600,000
Initial Revenue Stage [Member] \| Maximum [Member]
Investment [Line Items]
Revenue stage threshold											5,000,000
Expansion Stage [Member] \| Maximum [Member]
Investment [Line Items]
Revenue stage threshold											20,000,000
Expansion Stage [Member] \| Minimum [Member]
Investment [Line Items]
Revenue stage threshold											5,000,000
High Traction Stage [Member] \| Minimum [Member]
Investment [Line Items]
Revenue stage threshold											20,000,000
Good Start Genetics Inc [Member]
Investment [Line Items]
Number of shares received (in shares)		414,237
Number of shares received, amount held in escrow (in shares)		124,092
Gain on sale of business											3,800,000
Adjusted carrying value of capital												17,200,000	12,000,000
Beyond Com Inc [Member]
Investment [Line Items]
Gain on sale of business			$ 100,000
Consideration received			26,000,000
Proceeds from sale of business			$ 15,500,000
Term of note receivable (in years)			3 years
Amount of non-cash consideration received			$ 10,500,000.0
Stated interest rate			9.50%
Proceeds from divestiture of interest in joint venture			$ 15,500,000
Full Measure [Member]
Investment [Line Items]
Asset impairment charges											7,000,000
Adjusted carrying value of capital											0	8,600,000	4,000,000
Spongecell [Member]
Investment [Line Items]
Asset impairment charges											3,600,000
Adjusted carrying value of capital											6,000,000	18,000,000	14,000,000
Pneuron [Member]
Investment [Line Items]
Asset impairment charges											5,200,000
Adjusted carrying value of capital											0			$ 5,000,000
Putney, Inc. [Member]
Investment [Line Items]
Gain on sale of business											600,000	55,600,000
Proceeds from sale of business						$ 58,600,000
Amount held in escrow						600,000
Aventura [Member]
Investment [Line Items]
Asset impairment charges												3,600,000
Adjusted carrying value of capital											0	6,200,000	6,000,000
Penn Mezzanine Assets [Member]
Investment [Line Items]
Gain (loss) on extinguishment of debt											(399,000)	2,400,000
Other assets											$ 0	200,000
Appfirst [Member]
Investment [Line Items]
Proceeds from sale of business					$ 900,000
Asset impairment charges												1,700,000	3,600,000
Adjusted carrying value of capital													11,600,000
Bridgevine Inc [Member]
Investment [Line Items]
Gain on sale of business					400,000
Proceeds from sale of business					$ 5,000,000
DriveFactor Inc. [Member]
Investment [Line Items]
Gain on sale of business						1,100,000			$ 6,100,000
Proceeds from sale of business						1,100,000			$ 9,100,000
Thing Worx Inc [Member]
Investment [Line Items]
Gain on sale of business						3,300,000	$ 4,100,000	$ 3,300,000
Proceeds from sale of business						$ 3,300,000	$ 4,100,000	3,300,000
Quantia Inc. [Member]
Investment [Line Items]
Gain on sale of business				$ 600,000
Proceeds from sale of business				$ 600,000				7,800,000
Asset impairment charges													2,900,000
Alverix Inc [Member]
Investment [Line Items]
Gain on sale of business								1,700,000
Proceeds from sale of business								$ 1,700,000
Crescendo [Member]
Investment [Line Items]
Gain on sale of business										$ 2,900,000
Proceeds from sale of business										$ 2,900,000
InfoBionic [Member]
Investment [Line Items]
Asset impairment charges													3,200,000
Adjusted carrying value of capital												$ 14,500,000	9,500,000
Dabo [Member]
Investment [Line Items]
Asset impairment charges													$ 2,356,000
Subsequent Event [Member] \| Good Start Genetics Inc [Member]
Investment [Line Items]
Trading securities sold	414,237
Proceeds from sale of securities	$ 2,600,000