Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 86,317	$ 96,840
Securities sold not yet purchased
Contingent payment arrangements	17,384		$ 0
Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	115,022	102,888
Securities segregated (U.S. Treasury Bills)	958,149	1,169,554
Derivatives	14,710	17,079
Total investments measured at fair value and other measurements	323,205	714,344
Total assets measured at fair value and other measurements	1,411,086	2,003,865
Securities sold not yet purchased
Short equities – corporate	8,538	4,841
Short exchange-traded options	5,152	3,782
Derivatives	16,658	14,822
Contingent payment arrangements	26,432	7,336
Total liabilities measured at fair value	56,780	30,781
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	115,022	102,888
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	1,282	1,594
Total investments	149,142	216,251
Total assets measured at fair value	265,446	320,733
Securities sold not yet purchased
Short equities – corporate	8,538	4,841
Short exchange-traded options	5,152	3,782
Derivatives	240	2,534
Contingent payment arrangements	0	0
Total liabilities measured at fair value	13,930	11,157
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)	958,149	1,169,554
Derivatives	13,428	15,485
Total investments	87,319	400,796
Total assets measured at fair value	1,058,896	1,585,835
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	16,418	12,288
Contingent payment arrangements	0	0
Total liabilities measured at fair value	16,418	12,288
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
Total investments	118	142
Total assets measured at fair value	118	142
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	26,432	7,336
Total liabilities measured at fair value	26,432	7,336
Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
NAV Expedient	309	315
U.S. Treasury Bills | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	74,342	392,424
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	74,342	392,424
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	151,325	218,243
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	137,921	209,414
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	12,977	8,372
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	118	142
Equity securities | Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
NAV Expedient	309	315
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	7,674	7,358
Other investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	13,092
Total investments measured at fair value and other measurements		11,627
Other investments | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments	5,418	4,269
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	18,070	8,783
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	18,070	8,783
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5,803	2,568
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5,803	2,568
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]
Other	60,573	80,699
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 60,573	$ 80,699