Fair Value (Valuation of Financial Instruments) (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	$ 138,734	$ 107,250
U.S. Treasury Bills	1,206,774	922,126
Trading
Equity securities	156,214	153,962
Fixed income securities	90,136	91,580
Long exchange-traded options	1,558	3,106
Derivatives	7,768	8,413
Private equity	4,914	4,913
Available-for-sale	64	45
Total assets measured at fair value	1,606,162	1,291,395
Securities sold not yet purchased
Short equities – corporate	14,921	40,252
Short exchange-traded options	916	692
Derivatives	10,347	8,649
Contingent payment arrangements	17,177	17,589
Total liabilities measured at fair value	43,361	67,182
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	138,734	107,250
U.S. Treasury Bills	0	0
Trading
Equity securities	146,122	148,128
Fixed income securities	78,892	80,473
Long exchange-traded options	1,558	3,106
Derivatives	536	1,224
Private equity	0	0
Available-for-sale	64	45
Total assets measured at fair value	365,906	340,226
Securities sold not yet purchased
Short equities – corporate	14,921	40,252
Short exchange-traded options	916	692
Derivatives	1,909	1,092
Contingent payment arrangements	0	0
Total liabilities measured at fair value	17,746	42,036
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
U.S. Treasury Bills	1,206,774	922,126
Trading
Equity securities	9,980	5,724
Fixed income securities	11,244	11,107
Long exchange-traded options	0	0
Derivatives	7,232	7,189
Private equity	0	0
Available-for-sale	0	0
Total assets measured at fair value	1,235,230	946,146
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	8,438	7,557
Contingent payment arrangements	0	0
Total liabilities measured at fair value	8,438	7,557
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money markets	0	0
U.S. Treasury Bills	0	0
Trading
Equity securities	112	110
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	4,914	4,913
Available-for-sale	0	0
Total assets measured at fair value	5,026	5,023
Securities sold not yet purchased
Short equities – corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	17,177	17,589
Total liabilities measured at fair value	$ 17,177	$ 17,589