Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 82,218	$ 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	114,136	102,888
Securities segregated (U.S. Treasury Bills)	1,109,589	1,169,554
Derivatives	11,471	17,079
Total investments measured at fair value and other measurements	224,703	714,344
Total assets measured at fair value and other measurements	1,459,899	2,003,865
Securities sold not yet purchased
Short equities – corporate	3,227	4,841
Short exchange-traded options	2,177	3,782
Derivatives	21,329	14,822
Contingent payment arrangements	25,603	7,336
Total liabilities measured at fair value	52,336	30,781
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	114,136	102,888
Securities segregated (U.S. Treasury Bills)	0	0
Derivatives	220	1,594
Total investments	133,672	216,251
Total assets measured at fair value	248,028	320,733
Securities sold not yet purchased
Short equities – corporate	3,227	4,841
Short exchange-traded options	2,177	3,782
Derivatives	3,075	2,534
Contingent payment arrangements	0	0
Total liabilities measured at fair value	8,479	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)	1,109,589	1,169,554
Derivatives	11,251	15,485
Total investments	8,378	400,796
Total assets measured at fair value	1,129,218	1,585,835
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	18,254	12,288
Contingent payment arrangements	0	0
Total liabilities measured at fair value	18,254	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	117	142
Total assets measured at fair value	117	142
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	25,603	7,336
Total liabilities measured at fair value	25,603	7,336
Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Alternative Investment	318	315
U.S. Treasury Bills | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	0	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	0	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	134,820	218,243
Equity securities | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	126,007	209,414
Equity securities | Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	8,378	8,372
Equity securities | Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	117	142
Equity securities | Recurring | NAV Expedient
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Alternative Investment	318	315
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	6,900	7,358
Other investments | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	12,274
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	5,374	4,269
Time deposits
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	8,487	8,783
Time deposits | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	8,487	8,783
Long | Exchange-traded options | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	2,291	2,568
Long | Exchange-traded options | Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	2,291	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	66,831	80,699
Limited partnership hedge funds | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	$ 66,831	$ 80,699