Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	$ 39,900
Investments, Other Measurements	100,606	$ 118,149
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	173,176	107,250
Securities segregated (U.S. Treasury Bills)	1,007,079	893,189
Derivatives	7,876	8,413
Private equity	44,852	45,278
Investments, NAV Expedient	40,016	40,413
Available-for-sale	102	45
Total investments measured at fair value and other measurements	479,981	441,105
Total assets measured at fair value and other measurements	1,668,112	1,449,957
Securities sold not yet purchased
Short equities – corporate	15,042	40,252
Short exchange-traded options	7,510	692
Derivatives	8,176	8,649
Contingent payment arrangements	16,777	17,589
Total liabilities measured at fair value	47,505	67,182
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	173,176	107,250
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	813	1,224
Private equity	0	0
Available-for-sale	102	45
Total investments	291,196	231,752
Total assets measured at fair value	465,185	340,226
Securities sold not yet purchased
Short equities – corporate	15,042	40,252
Short exchange-traded options	7,510	692
Derivatives	916	1,092
Contingent payment arrangements	0	0
Total liabilities measured at fair value	23,468	42,036
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	1,007,079	893,189
Derivatives	7,063	7,189
Private equity	0	0
Available-for-sale	0	0
Total investments	43,135	45,768
Total assets measured at fair value	1,057,277	946,146
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	7,260	7,557
Contingent payment arrangements	0	0
Total liabilities measured at fair value	7,260	7,557
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	0	0
Private equity	4,914	4,913
Available-for-sale	0	0
Total investments	5,028	5,023
Total assets measured at fair value	5,028	5,023
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	16,777	17,589
Total liabilities measured at fair value	16,777	17,589
U.S. Treasury Bills | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	27,764	28,937
U.S. Treasury Bills | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
U.S. Treasury Bills | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	27,764	28,937
U.S. Treasury Bills | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Equity securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	208,603	153,998
Investments, NAV Expedient	65	36
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	204,418	148,128
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	4,006	5,724
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	114	110
Fixed income securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	89,706	91,592
Investments, NAV Expedient	13	12
Fixed income securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	78,328	80,473
Fixed income securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	11,365	11,107
Fixed income securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Private equity | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	39,938	40,365
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	11,498	7,522
Other investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	11,498	7,522
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	50,429	70,097
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	50,429	70,097
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	8,348	3,106
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	8,348	3,106
Long | Exchange-traded options | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Long | Exchange-traded options | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	0
Limited partnership hedge funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	38,679	40,530
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	$ 38,679	$ 40,530