Fair Value (Details) (USD $)	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Trading - Common stocks [Member]	Jun. 30, 2011 Trading - Common stocks [Member]	Jun. 30, 2012 Trading - Common stocks [Member]	Jun. 30, 2011 Trading - Common stocks [Member]	Jun. 30, 2012 Trading - Other [Member]	Jun. 30, 2011 Trading - Other [Member]	Jun. 30, 2012 Trading - Other [Member]	Jun. 30, 2011 Trading - Other [Member]	Jun. 30, 2012 Recurring Basis [Member]	Dec. 31, 2011 Recurring Basis [Member]	Jun. 30, 2011 Recurring Basis [Member]	Jun. 30, 2012 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] Recurring Basis [Member]	Dec. 31, 2011 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] Recurring Basis [Member]	Jun. 30, 2011 Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] Recurring Basis [Member]	Jun. 30, 2012 Significant Other Observable Inputs (Level 2) [Member] Recurring Basis [Member]	Dec. 31, 2011 Significant Other Observable Inputs (Level 2) [Member] Recurring Basis [Member]	Jun. 30, 2011 Significant Other Observable Inputs (Level 2) [Member] Recurring Basis [Member]	Jun. 30, 2012 Significant Unobservable Inputs (Level 3) [Member] Recurring Basis [Member]	Dec. 31, 2011 Significant Unobservable Inputs (Level 3) [Member] Recurring Basis [Member]	Jun. 30, 2011 Significant Unobservable Inputs (Level 3) [Member] Recurring Basis [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notice period to redeem investments in funds (in days)			30D
Assets
Cash equivalents													$ 324,156,000	$ 260,969,000	$ 260,177,000	$ 324,156,000	$ 260,969,000	$ 260,177,000	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Investments in partnerships													23,704,000	27,122,000	27,977,000	0	0	0	23,704,000	27,122,000	27,977,000	0	0	0
Investments in securities:
AFS-Common stocks													32,815,000	33,282,000	35,296,000	32,815,000	33,282,000	35,296,000	0	0	0	0	0	0
AFS-Mutual Funds													2,013,000	1,905,000	2,098,000	2,013,000	1,905,000	2,098,000	0	0	0	0	0	0
Trading-Government obligations													50,139,000	42,126,000	7,925,000	50,139,000	42,126,000	7,925,000	0	0	0	0	0	0
Trading-Common Stocks													162,883,000	159,314,000	251,118,000	161,956,000	158,623,000	250,524,000	256,000	21,000	10,000	671,000	670,000	584,000
Trading-Mutual Funds													1,406,000	1,307,000	1,501,000	1,406,000	1,307,000	1,501,000	0	0	0	0	0	0
Trading-Other													374,000	399,000	487,000	23,000	55,000	118,000	0	60,000	0	351,000	284,000	369,000
Total investments in securities													249,630,000	238,333,000	298,425,000	248,352,000	237,298,000	297,462,000	256,000	81,000	10,000	1,022,000	954,000	953,000
Investments in sponsored registered investment companies: [Abstract]
AFS - Closed-end Funds													56,171,000	55,855,000	60,900,000	56,171,000	55,855,000	60,900,000	0	0	0	0	0	0
AFS - Mutual Funds													3,371,000	3,341,000	3,956,000	3,371,000	3,341,000	3,956,000	0	0	0	0	0	0
Trading - Mutual Funds													19,000	18,000	24,000	19,000	18,000	24,000	0	0	0	0	0	0
Total investments in sponsored registered investment companies													59,561,000	59,214,000	64,880,000	59,561,000	59,214,000	64,880,000	0	0	0	0	0	0
Total investments													332,895,000	324,669,000	391,282,000	307,913,000	296,512,000	362,342,000	23,960,000	27,203,000	27,987,000	1,022,000	954,000	953,000
Total assets at fair value													657,051,000	585,638,000	651,459,000	632,069,000	557,481,000	622,519,000	23,960,000	27,203,000	27,987,000	1,022,000	954,000	953,000
Liabilities
Trading-Common Stocks													6,687,000	5,415,000	10,238,000	6,687,000	5,415,000	10,238,000	0	0	0	0	0	0
Trading-Other													323,000	73,000	6,000	0	0	6,000	323,000	73,000	0	0	0	0
Securities sold, not yet purchased													7,010,000	5,488,000	10,244,000	6,687,000	5,415,000	10,244,000	323,000	73,000	0	0	0	0
Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis [Roll Forward]
Beginning Balance	925,000	924,000	954,000	425,000	647,000	568,000	670,000	147,000	278,000	356,000	284,000	278,000
Total Realized and Unrealized Gains or (Losses) in Income -Trading	83,000	13,000	81,000	160,000	24,000	16,000	24,000	37,000	59,000	(3,000)	57,000	123,000
Total Realized and Unrealized Gains or (Losses) in Income - AFS Investments	0	0	0	0	0	0	0	0	0	0	0	0
Total Unrealized Gains or (Losses) Included in Other Comprehensive Income	0	0	0	0	0	0	0	0	0	0	0	0
Total Realized and Unrealized Gains or (Losses)	83,000	13,000	81,000	160,000	24,000	16,000	24,000	37,000	59,000	(3,000)	57,000	123,000
Purchases	14,000	24,000	75,000	424,000	0	14,000	57,000	414,000	14,000	10,000	18,000	10,000
Sales	0	(14,000)	(88,000)	(62,000)	0	(14,000)	(80,000)	(14,000)	0	0	(8,000)	(48,000)
Transfers In and/or (Out) of Level 3	0	6,000	0	6,000	0	0	0	0	0	6,000	0	6,000
Ending Balance	1,022,000	953,000	1,022,000	953,000	671,000	584,000	671,000	584,000	351,000	369,000	351,000	369,000
Reclassification of investment from Level 1 to Level 3		$ 6,000		$ 6,000