Fair Value Measurements for Operating Entities and Consolidated Funds Unobservable Input Roll Forward (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		$ 15,811,000		$ 12,872,000
Asset, Transfers In		0		0
Asset, Transfers Out		(25,098,000)	[1],[2]	(1,000,000)	[3]
Purchases/(covers)		25,785,000		3,717,000
(Sales)/short buys		(2,956,000)		(218,000)
Realized and unrealized gains (losses), asset		(5,427,000)		440,000
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	486,000		34,000
Balance Asset Value		8,115,000		15,811,000
Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		10,121,000		2,278,000
Asset, Transfers In		0		0
Asset, Transfers Out		0		0
Purchases/(covers)		2,670,000		7,099,000
(Sales)/short buys		(2,688,000)		(3,222,000)
Realized and unrealized gains (losses), asset		(2,533,000)		3,966,000
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	(412,000)		3,972,000
Balance Asset Value		7,570,000		10,121,000
Convertible Bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		250,000		819,000
Asset, Transfers In		0		0
Asset, Transfers Out		0		0
Purchases/(covers)		282,000		0
(Sales)/short buys		0		(569,000)
Realized and unrealized gains (losses), asset		(250,000)		0
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	0		0
Balance Asset Value		282,000		250,000
Options
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		14,753,000		18,194,000
Asset, Transfers In		0		0
Asset, Transfers Out		0		0
Purchases/(covers)		0		0
(Sales)/short buys		0		0
Realized and unrealized gains (losses), asset		(13,298,000)		(3,441,000)
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	(13,298,000)		(3,441,000)
Balance Asset Value		1,455,000		14,753,000
Warrants and Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		3,719,000		2,572,000
Asset, Transfers In		2,173,000	[5],[6]	0
Asset, Transfers Out		0		0
Purchases/(covers)		0		1,914,000
(Sales)/short buys		(4,118,000)		(817,000)
Realized and unrealized gains (losses), asset		743,000		50,000
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	510,000		79,000
Balance Asset Value		2,517,000		3,719,000
Trade Claims
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		562,000		0
Asset, Transfers In		0		675,000	[7]
Asset, Transfers Out		0		0
Purchases/(covers)		5,865,000		289,000
(Sales)/short buys		(611,000)		(325,000)
Realized and unrealized gains (losses), asset		134,000		(77,000)
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	202,000		(111,000)
Balance Asset Value		5,950,000		562,000
Lehman claims, at fair value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		265,000		299,000
Asset, Transfers In		0		0
Asset, Transfers Out		0		0
Purchases/(covers)		0		0
(Sales)/short buys		0		0
Realized and unrealized gains (losses), asset		36,000		(34,000)
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	36,000		(35,000)
Balance Asset Value		301,000		265,000
Consolidated Funds | Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		36,928,000		32,000,000
Asset, Transfers In		22,644,000	[1]	0
Asset, Transfers Out	[2]	(13,668,000)		(11,000,000)
Purchases/(covers)		8,437,000		13,483,000
(Sales)/short buys		(11,903,000)		0
Realized and unrealized gains (losses), asset		8,007,000		2,445,000
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	3,896,000		2,445,000
Balance Asset Value		50,445,000		36,928,000
Consolidated Funds | Common Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		295,000		0
Asset, Transfers In		0		0
Asset, Transfers Out		0		0
Purchases/(covers)		526,000		314,000
(Sales)/short buys		(848,000)		0
Realized and unrealized gains (losses), asset		77,000		(19,000)
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	0		(19,000)
Balance Asset Value		50,000		295,000
Consolidated Funds | Warrants and Rights
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		3,000		0
Asset, Transfers In		0		0
Asset, Transfers Out		0		0
Purchases/(covers)		0		0
(Sales)/short buys		0		0
Realized and unrealized gains (losses), asset		3,565,000		3,000
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	3,565,000		3,000
Balance Asset Value		3,568,000		3,000
Consolidated Funds | Loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Asset Value		657,000		0
Asset, Transfers In		0		0
Asset, Transfers Out		0		0
Purchases/(covers)		202,000		590,000
(Sales)/short buys		(1,021,000)		0
Realized and unrealized gains (losses), asset		162,000		67,000
Change in Unrealized Gain (Loss), instruments still held, asset	[4]	0		0
Balance Asset Value		0		657,000
Options, liability
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value		14,753,000		18,194,000
Liability, Transfers In		0		0
Liability, Transfers Out		0		0
Liability, Purchases		0		0
Liability, Sales		0		0
Realized and unrealized gains (losses), liability		7,648,000		(3,441,000)
Change in Unrealized Gain (Loss), instruments still held, liabilities	[4]	7,648,000		(3,441,000)
Balance Liability Value		22,401,000		14,753,000
Contingent liability payable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance Liability Value		5,997,000		6,158,000
Liability, Transfers In		0		0
Liability, Transfers Out		0		0
Liability, Purchases		0		2,397,000
Liability, Sales		(392,000)		(4,697,000)
Realized and unrealized gains (losses), liability		(2,165,000)		2,139,000
Change in Unrealized Gain (Loss), instruments still held, liabilities	[4]	$ (2,165,000)		0
Balance Liability Value				$ 5,997,000

[1] The Company transferred investments to a consolidated fund.

[2] The investments were converted to common stock.

[3] The entity in which the Company is invested completed an initial public offering.

[4] Unrealized gains/losses are reported in other income (loss) in the accompanying consolidated statements of operations.

[5] As part of the preferred stock sale, the sellers received contingent value rights to be paid in the event certain milestones are reached.

[6] The Company received warrants as part of a transaction.

[7] The investment undertook a reorganization and subsequently is not traded in an active market.