Fair Value (Valuation of Financial Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Securities sold not yet purchased [Abstract]
Contingent payment arrangements	$ 0		$ 9,365
Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	254,133	$ 89,566
U.S. Treasury bills	459,007	444,152
Available-for-sale [Abstract]
Equity securities	5,527	5,951
Fixed income securities	298	221
Trading [Abstract]
Equity securities	354,244	387,502
Fixed income securities	180,924	167,059
Long exchange-traded options	7,935	22,290
Derivatives	14,569	6,787
Private equity	65,907	71,088
Total assets measured at fair value	1,342,544	1,194,616
Securities sold not yet purchased [Abstract]
Short equities - corporate	20,642	81,784
Short exchange-traded options	1,907	7,118
Derivatives	10,615	9,392
Contingent payment arrangements	42,032	42,436
Total liabilities measured at fair value	75,196	140,730
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	254,133	89,566
U.S. Treasury bills	0	0
Available-for-sale [Abstract]
Equity securities	5,527	5,951
Fixed income securities	298	221
Trading [Abstract]
Equity securities	331,417	387,495
Fixed income securities	178,203	164,317
Long exchange-traded options	7,935	22,290
Derivatives	2,347	571
Private equity	15,177	12,162
Total assets measured at fair value	795,037	682,573
Securities sold not yet purchased [Abstract]
Short equities - corporate	20,642	81,784
Short exchange-traded options	1,907	7,118
Derivatives	168	2,438
Contingent payment arrangements	0	0
Total liabilities measured at fair value	22,717	91,340
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
U.S. Treasury bills	459,007	444,152
Available-for-sale [Abstract]
Equity securities	0	0
Fixed income securities	0	0
Trading [Abstract]
Equity securities	22,703	7
Fixed income securities	2,721	2,742
Long exchange-traded options	0	0
Derivatives	12,222	6,216
Private equity	0	0
Total assets measured at fair value	496,653	453,117
Securities sold not yet purchased [Abstract]
Short equities - corporate	0	0
Short exchange-traded options	0	0
Derivatives	10,447	6,954
Contingent payment arrangements	0	0
Total liabilities measured at fair value	10,447	6,954
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Money markets	0	0
U.S. Treasury bills	0	0
Available-for-sale [Abstract]
Equity securities	0	0
Fixed income securities	0	0
Trading [Abstract]
Equity securities	124	0
Fixed income securities	0	0
Long exchange-traded options	0	0
Derivatives	0	0
Private equity	50,730	58,926
Total assets measured at fair value	50,854	58,926
Securities sold not yet purchased [Abstract]
Short equities - corporate	0	0
Short exchange-traded options	0	0
Derivatives	0	0
Contingent payment arrangements	42,032	42,436
Total liabilities measured at fair value	$ 42,032	$ 42,436