Income Taxes - Deferred Tax Assets and Deferred Tax Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax asset:
Carrying values of partner companies and other holdings	$ 69,751	$ 95,134
Tax loss and credit carryforwards	58,138	82,775
Accrued expenses	766	1,183
Stock-based compensation	814	1,763
Other	967	1,310
Deferred tax assets, gross	130,436	182,165
Valuation allowance	(130,436)	(182,165)
Net deferred tax asset	$ 0	$ 0