Fair Value (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Assets
Investments in partnerships	$ 24,094	$ 23,146	$ 24,094	$ 23,146	$ 25,253
Liabilities
Securities sold, not yet purchased	14,180	7,725	14,180	7,725	6,178
Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis [Roll Forward]
Beginning Balance	1,010	953	984	1,037
Total Realized and Unrealized Gains or (Losses) - Trading	8	(3)	24	(11)
Total Realized and Unrealized Gains or (Losses) in Income - AFS Investments	0	0	0	0
Total Unrealized Gains or (Losses) Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Gains or (Losses)	8	(3)	24	(11)
Purchases	0	0	10	3
Sales	0	0	0	79
Transfers In and/or (Out) of Level 3	0	0	0	0
Ending Balance	1,018	950	1,018	950
Trading - Common stocks [Member]
Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis [Roll Forward]
Beginning Balance	716	669	700	675
Total Realized and Unrealized Gains or (Losses) - Trading	8	(2)	24	(8)
Total Realized and Unrealized Gains or (Losses) in Income - AFS Investments	0	0	0	0
Total Unrealized Gains or (Losses) Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Gains or (Losses)	8	(2)	24	(8)
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers In and/or (Out) of Level 3	0	0	0	0
Ending Balance	724	667	724	667
Trading - Other [Member]
Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis [Roll Forward]
Beginning Balance	294	284	284	362
Total Realized and Unrealized Gains or (Losses) - Trading	0	(1)	0	(3)
Total Realized and Unrealized Gains or (Losses) in Income - AFS Investments	0	0	0	0
Total Unrealized Gains or (Losses) Included in Other Comprehensive Income	0	0	0	0
Total Realized and Unrealized Gains or (Losses)	0	(1)	0	(3)
Purchases	0	0	10	3
Sales	0	0	0	79
Transfers In and/or (Out) of Level 3	0	0	0	0
Ending Balance	294	283	294	283
Recurring Basis [Member]
Assets
Cash equivalents	358,210	244,144	358,210	244,144	209,913
Investments in partnerships	24,094	23,146	24,094	23,146	25,253
Investments in securities:
AFS-Common stocks	36,380	36,297	36,380	36,297	43,046
AFS-Mutual Funds	1,389	1,575	1,389	1,575	1,687
Trading-Government obligations	20,999	21,000	20,999	21,000	37,994
Trading-Common Stocks	190,939	166,443	190,939	166,443	124,634
Trading-Mutual Funds	3,373	12,010	3,373	12,010	23,285
Trading-Other	1,550	419	1,550	419	582
Total investments in securities	254,630	237,744	254,630	237,744	231,228
Investments in sponsored registered investment companies: [Abstract]
AFS - Closed-end Funds	36,142	40,272	36,142	40,272	40,624
AFS - Mutual Funds	3,377	3,404	3,377	3,404	3,408
Trading - Mutual Funds	1	12	1	12	10
Total investments in sponsored registered investment companies	39,520	43,688	39,520	43,688	44,042
Total investments	318,244	304,578	318,244	304,578	300,523
Total assets at fair value	676,454	548,722	676,454	548,722	510,436
Liabilities
Trading-Common Stocks	13,514	7,003	13,514	7,003	6,023
Trading-Other	666	722	666	722	155
Securities sold, not yet purchased	14,180	7,725	14,180	7,725	6,178
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring Basis [Member]
Assets
Cash equivalents	358,210	244,144	358,210	244,144	209,913
Investments in partnerships	0	0	0	0	0
Investments in securities:
AFS-Common stocks	36,380	36,297	36,380	36,297	43,046
AFS-Mutual Funds	1,389	1,575	1,389	1,575	1,687
Trading-Government obligations	20,999	21,000	20,999	21,000	37,994
Trading-Common Stocks	190,215	165,776	190,215	165,776	123,927
Trading-Mutual Funds	3,373	12,010	3,373	12,010	23,285
Trading-Other	453	32	453	32	23
Total investments in securities	252,809	236,690	252,809	236,690	229,962
Investments in sponsored registered investment companies: [Abstract]
AFS - Closed-end Funds	36,142	40,272	36,142	40,272	40,624
AFS - Mutual Funds	3,377	3,404	3,377	3,404	3,408
Trading - Mutual Funds	1	12	1	12	10
Total investments in sponsored registered investment companies	39,520	43,688	39,520	43,688	44,042
Total investments	292,329	280,378	292,329	280,378	274,004
Total assets at fair value	650,539	524,522	650,539	524,522	483,917
Liabilities
Trading-Common Stocks	13,514	7,003	13,514	7,003	6,023
Trading-Other	0	0	0	0	0
Securities sold, not yet purchased	13,514	7,003	13,514	7,003	6,023
Significant Other Observable Inputs (Level 2) [Member] | Recurring Basis [Member]
Assets
Cash equivalents	0	0	0	0	0
Investments in partnerships	24,094	23,146	24,094	23,146	25,253
Investments in securities:
AFS-Common stocks	0	0	0	0	0
AFS-Mutual Funds	0	0	0	0	0
Trading-Government obligations	0	0	0	0	0
Trading-Common Stocks	0	0	0	0	7
Trading-Mutual Funds	0	0	0	0	0
Trading-Other	803	104	803	104	275
Total investments in securities	803	104	803	104	282
Investments in sponsored registered investment companies: [Abstract]
AFS - Closed-end Funds	0	0	0	0	0
AFS - Mutual Funds	0	0	0	0	0
Trading - Mutual Funds	0	0	0	0	0
Total investments in sponsored registered investment companies	0	0	0	0	0
Total investments	24,897	23,250	24,897	23,250	25,535
Total assets at fair value	24,897	23,250	24,897	23,250	25,535
Liabilities
Trading-Common Stocks	0	0	0	0	0
Trading-Other	666	722	666	722	155
Securities sold, not yet purchased	666	722	666	722	155
Significant Unobservable Inputs (Level 3) [Member] | Recurring Basis [Member]
Assets
Cash equivalents	0	0	0	0	0
Investments in partnerships	0	0	0	0	0
Investments in securities:
AFS-Common stocks	0	0	0	0	0
AFS-Mutual Funds	0	0	0	0	0
Trading-Government obligations	0	0	0	0	0
Trading-Common Stocks	724	667	724	667	700
Trading-Mutual Funds	0	0	0	0	0
Trading-Other	294	283	294	283	284
Total investments in securities	1,018	950	1,018	950	984
Investments in sponsored registered investment companies: [Abstract]
AFS - Closed-end Funds	0	0	0	0	0
AFS - Mutual Funds	0	0	0	0	0
Trading - Mutual Funds	0	0	0	0	0
Total investments in sponsored registered investment companies	0	0	0	0	0
Total investments	1,018	950	1,018	950	984
Total assets at fair value	1,018	950	1,018	950	984
Liabilities
Trading-Common Stocks	0	0	0	0	0
Trading-Other	0	0	0	0	0
Securities sold, not yet purchased	$ 0	$ 0	$ 0	$ 0	$ 0