Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, NAV Expedient	$ 37,600
Investments, Other Measurements	103,583	$ 118,149
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	152,893	107,250
Securities segregated (U.S. Treasury Bills)	723,029	893,189
Derivatives	5,846	8,413
Private equity	38,553	45,278
Investments, NAV Expedient	37,710	40,413
Available-for-sale	98	45
Total investments measured at fair value and other measurements	519,753	441,105
Total assets measured at fair value and other measurements	1,401,521	1,449,957
Securities sold not yet purchased
Short equities – corporate	20,539	40,252
Short exchange-traded options	13,272	692
Derivatives	6,600	8,649
Contingent payment arrangements	12,103	17,589
Total liabilities measured at fair value	52,514	67,182
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	152,893	107,250
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1,111	1,224
Private equity	0	0
Available-for-sale	98	45
Total investments	309,691	231,752
Total assets measured at fair value	463,695	340,226
Securities sold not yet purchased
Short equities – corporate	20,539	40,252
Short exchange-traded options	13,272	692
Derivatives	962	1,092
Contingent payment arrangements	0	0
Total liabilities measured at fair value	34,773	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)	723,029	893,189
Derivatives	4,735	7,189
Private equity	0	0
Available-for-sale	0	0
Total investments	67,699	45,768
Total assets measured at fair value	795,463	946,146
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	5,638	7,557
Contingent payment arrangements	0	0
Total liabilities measured at fair value	5,638	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	954	4,913
Available-for-sale	0	0
Total investments	1,070	5,023
Total assets measured at fair value	1,070	5,023
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	12,103	17,589
Total liabilities measured at fair value	12,103	17,589
U.S. Treasury Bills | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	52,605	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	52,605	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	236,219	153,998
Investments, NAV Expedient	98	36
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	231,897	148,128
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	4,108	5,724
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	116	110
Fixed income securities | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	79,327	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	68,328	80,473
Fixed income securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	10,986	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	37,599	40,365
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	12,003	7,522
Other investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	12,003	7,522
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	52,090	70,097
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	52,090	70,097
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	9,368	3,106
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	9,368	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	39,490	40,530
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Other Measurements	$ 39,490	$ 40,530