Fair Value - Valuation of Financial Instruments by Pricing Observability Levels (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Securities sold not yet purchased
Contingent payment arrangements	$ 11,893	$ 0	$ 9,365
Consolidated VIEs [Member]
Trading
Derivatives	4,997
Securities sold not yet purchased
Gross Amounts of Recognized Liabilities	2,081
Derivatives	2,081
Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	107,250	116,445
U.S. Treasury Bills	922,126	485,121
Available-for-sale
Equity securities	23	181
Fixed income securities	22	183
Trading
Equity securities	176,211	326,235
Fixed income securities	91,580	182,776
Long exchange-traded options	3,106	5,910
Derivatives	8,413	12,588
Private equity	4,913	30,340
Total assets measured at fair value	1,866,209	1,159,779
Trading Liabilities, Fair Value Disclosure	40,252	15,254
Securities sold not yet purchased
Gross Amounts of Recognized Liabilities	692	843
Derivatives	8,649	12,221
Contingent payment arrangements	17,589	31,399
Total liabilities measured at fair value	317,682	59,717
Fair Value, Measurements, Recurring [Member] \| Consolidated VIEs [Member]
Securities sold not yet purchased
Derivatives	2,081
Short equities	248,419
Fair Value, Measurements, Recurring [Member] \| Equity Securities [Member] \| Consolidated VIEs [Member]
Trading
Investments	550,768
Fair Value, Measurements, Recurring [Member] \| Derivatives [Member] \| Consolidated VIEs [Member]
Trading
Investments	1,797
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	107,250	116,445
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	23	181
Fixed income securities	22	183
Trading
Equity securities	158,316	325,248
Fixed income securities	80,473	170,244
Long exchange-traded options	3,106	5,910
Derivatives	1,224	1,539
Private equity	0	14,305
Total assets measured at fair value	692,302	634,055
Trading Liabilities, Fair Value Disclosure	40,252	15,254
Securities sold not yet purchased
Gross Amounts of Recognized Liabilities	692	843
Derivatives	1,092	2,651
Contingent payment arrangements	0	0
Total liabilities measured at fair value	290,503	18,748
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Consolidated VIEs [Member]
Securities sold not yet purchased
Derivatives	48
Short equities	248,419
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Equity Securities [Member] \| Consolidated VIEs [Member]
Trading
Investments	341,830
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member] \| Derivatives [Member] \| Consolidated VIEs [Member]
Trading
Investments	58
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
U.S. Treasury Bills	922,126	485,121
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	17,785	874
Fixed income securities	11,107	12,532
Long exchange-traded options	0	0
Derivatives	7,189	11,049
Private equity	0	0
Total assets measured at fair value	1,163,143	509,576
Trading Liabilities, Fair Value Disclosure	0	0
Securities sold not yet purchased
Gross Amounts of Recognized Liabilities	0	0
Derivatives	7,557	9,570
Contingent payment arrangements	0	0
Total liabilities measured at fair value	9,590	9,570
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Consolidated VIEs [Member]
Securities sold not yet purchased
Derivatives	2,033
Short equities	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Equity Securities [Member] \| Consolidated VIEs [Member]
Trading
Investments	203,197
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Derivatives [Member] \| Consolidated VIEs [Member]
Trading
Investments	1,739
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
U.S. Treasury Bills	0	0
Available-for-sale
Equity securities	0	0
Fixed income securities	0	0
Trading
Equity securities	110	113
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	4,913	16,035
Total assets measured at fair value	10,764	16,148
Trading Liabilities, Fair Value Disclosure	0	0
Securities sold not yet purchased
Gross Amounts of Recognized Liabilities	0	0
Derivatives	0	0
Contingent payment arrangements	17,589	31,399
Total liabilities measured at fair value	17,589	$ 31,399
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Consolidated VIEs [Member]
Securities sold not yet purchased
Derivatives	0
Short equities	0
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Equity Securities [Member] \| Consolidated VIEs [Member]
Trading
Investments	5,741
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Derivatives [Member] \| Consolidated VIEs [Member]
Trading
Investments	$ 0

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. This element represents the fair value of assets categorized as long-exchange traded options.

No definition available.