Financial Instruments - Items Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	$ 14,036	$ 57,698
Financial instruments held for investment activities	14,285	3,461
Total	28,321	61,159
Investment funds valued at net asset value	66,602	104,888
Total financial instruments owned, at fair value	94,923	166,047
Financial instruments sold, not yet purchased, at fair value	1,934	121,310
Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	13,221	14,832
Financial instruments sold, not yet purchased, at fair value	1,933	34,043
Convertible and Fixed Income Debt Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	815	42,864
Financial instruments sold, not yet purchased, at fair value	1	2,317
Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary		2
Marketable Equity Securities [Member] | Available-for-Sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities		172
U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value		84,950
Estimated Fair Value, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	5,586	14,760
Financial instruments held for investment activities	5,415	347
Total	11,001	15,107
Investment funds valued at net asset value	0	0
Total financial instruments owned, at fair value	0	0
Financial instruments sold, not yet purchased, at fair value	1,933	118,993
Estimated Fair Value, Level 1 [Member] | Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	5,586	14,758
Financial instruments sold, not yet purchased, at fair value	1,933	34,043
Estimated Fair Value, Level 1 [Member] | Convertible and Fixed Income Debt Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	0	0
Financial instruments sold, not yet purchased, at fair value	0	0
Estimated Fair Value, Level 1 [Member] | Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary		2
Estimated Fair Value, Level 1 [Member] | Marketable Equity Securities [Member] | Available-for-Sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities		172
Estimated Fair Value, Level 1 [Member] | U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value		84,950
Estimated Fair Value, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	815	42,864
Financial instruments held for investment activities	0	0
Total	815	42,864
Investment funds valued at net asset value	0	0
Total financial instruments owned, at fair value	0	0
Financial instruments sold, not yet purchased, at fair value	1	2,317
Estimated Fair Value, Level 2 [Member] | Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	0	0
Financial instruments sold, not yet purchased, at fair value	0	0
Estimated Fair Value, Level 2 [Member] | Convertible and Fixed Income Debt Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	815	42,864
Financial instruments sold, not yet purchased, at fair value	1	2,317
Estimated Fair Value, Level 2 [Member] | Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary		0
Estimated Fair Value, Level 2 [Member] | Marketable Equity Securities [Member] | Available-for-Sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities		0
Estimated Fair Value, Level 2 [Member] | U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value		0
Estimated Fair Value, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	7,635	74
Financial instruments held for investment activities	8,870	3,114
Total	16,505	3,188
Investment funds valued at net asset value	0	0
Total financial instruments owned, at fair value	0	0
Financial instruments sold, not yet purchased, at fair value	0	0
Estimated Fair Value, Level 3 [Member] | Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	7,635	74
Financial instruments sold, not yet purchased, at fair value	0	0
Estimated Fair Value, Level 3 [Member] | Convertible and Fixed Income Debt Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary	0	0
Financial instruments sold, not yet purchased, at fair value	0	0
Estimated Fair Value, Level 3 [Member] | Listed Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for trading activities at broker-dealer subsidiary		0
Estimated Fair Value, Level 3 [Member] | Marketable Equity Securities [Member] | Available-for-Sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities		0
Estimated Fair Value, Level 3 [Member] | U.S. Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, not yet purchased, at fair value		0
Trading Securities [Member] | Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	13,849	2,325
Trading Securities [Member] | Warrants [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	436	964
Trading Securities [Member] | Estimated Fair Value, Level 1 [Member] | Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	5,415	175
Trading Securities [Member] | Estimated Fair Value, Level 1 [Member] | Warrants [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	0	0
Trading Securities [Member] | Estimated Fair Value, Level 2 [Member] | Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	0	0
Trading Securities [Member] | Estimated Fair Value, Level 2 [Member] | Warrants [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	0	0
Trading Securities [Member] | Estimated Fair Value, Level 3 [Member] | Marketable and Non-Public Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	8,434	2,150
Trading Securities [Member] | Estimated Fair Value, Level 3 [Member] | Warrants [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments held for investment activities	$ 436	$ 964