Fair Value (Details) - Major Types of Debt and Equity Securities [Domain] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Assets
Investments in partnerships	$ 21,526	$ 24,856	$ 21,526	$ 24,856	$ 23,803
Liabilities
Securities sold, not yet purchased	9,825	14,329	9,825	14,329	10,595
Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis [Roll Forward]
Beginning Balance	1,218	984	1,587	984
Total Realized and Unrealized Gains or (Losses) - Trading	(1)	16	62	16
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)	(1)	16	62	16
Purchases	3	10	11	10
Sales	(2)	0	442	0
Transfers In and/or (Out) of Level 3	0	0	0	0
Ending Balance	1,218	1,010	1,218	1,010
Recurring Basis [Member]
Assets
Cash equivalents	372,079	307,154	372,079	307,154	297,971
Investments in partnerships	21,526	24,855	21,526	24,855	23,803
Investments in securities:
AFS-Common stocks	37,429	39,334	37,429	39,334	38,942
AFS-Mutual Funds	1,293	1,516	1,293	1,516	1,368
Trading-Government obligations	7,000	23,998	7,000	23,998	18,996
Trading-Common Stocks	93,874	156,522	93,874	156,522	195,029
Trading-Mutual Funds	3,623	3,532	3,623	3,532	3,498
Trading-Other	789	694	789	694	1,704
Total investments in securities	144,008	225,596	144,008	225,596	259,537
Investments in sponsored registered investment companies: [Abstract]
AFS - Closed-end Funds	77,616	37,138	77,616	37,138	36,323
AFS - Mutual Funds	3,283	3,639	3,283	3,639	3,213
Trading - Mutual Funds	45,406	14	45,406	14	1
Total investments in sponsored registered investment companies	126,305	40,791	126,305	40,791	39,537
Total investments	291,839	291,242	291,839	291,242	322,877
Total assets at fair value	663,918	598,396	663,918	598,396	620,848
Liabilities
Trading-Common Stocks	9,715	13,861	9,715	13,861	9,960
Trading-Other	110	468	110	468	635
Securities sold, not yet purchased	9,825	14,329	9,825	14,329	10,595
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring Basis [Member]
Assets
Cash equivalents	372,079	307,154	372,079	307,154	297,971
Investments in partnerships	0	0	0	0	0
Investments in securities:
AFS-Common stocks	37,429	39,334	37,429	39,334	38,942
AFS-Mutual Funds	1,293	1,516	1,293	1,516	1,368
Trading-Government obligations	7,000	23,998	7,000	23,998	18,996
Trading-Common Stocks	92,954	155,805	92,954	155,805	193,735
Trading-Mutual Funds	3,623	3,532	3,623	3,532	3,498
Trading-Other	318	243	318	243	513
Total investments in securities	142,617	224,428	142,617	224,428	257,052
Investments in sponsored registered investment companies: [Abstract]
AFS - Closed-end Funds	77,616	37,138	77,616	37,138	36,323
AFS - Mutual Funds	3,283	3,639	3,283	3,639	3,213
Trading - Mutual Funds	45,406	14	45,406	14	1
Total investments in sponsored registered investment companies	126,305	40,791	126,305	40,791	39,537
Total investments	268,922	265,219	268,922	265,219	296,589
Total assets at fair value	641,001	572,373	641,001	572,373	594,560
Liabilities
Trading-Common Stocks	9,715	13,861	9,715	13,861	9,960
Trading-Other	0	0	0	0	0
Securities sold, not yet purchased	9,715	13,861	9,715	13,861	9,960
Significant Other Observable Inputs (Level 2) [Member] | Recurring Basis [Member]
Assets
Cash equivalents	0	0	0	0	0
Investments in partnerships	21,526	24,855	21,526	24,855	23,803
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	1	0	1	1
Trading-Mutual Funds	0	0	0	0	0
Trading-Other	173	157	173	157	897
Total investments in securities	173	158	173	158	898
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	21,699	25,013	21,699	25,013	24,701
Total assets at fair value	21,699	25,013	21,699	25,013	24,701
Liabilities
Trading-Common Stocks	0	0	0	0	0
Trading-Other	110	468	110	468	635
Securities sold, not yet purchased	110	468	110	468	635
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	920	716	920	716	1,293
Trading-Mutual Funds	0	0	0	0	0
Trading-Other	298	294	298	294	294
Total investments in securities	1,218	1,010	1,218	1,010	1,587
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	1,218	1,010	1,218	1,010	1,587
Total assets at fair value	1,218	1,010	1,218	1,010	1,587
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
Investment in Sponsored Registered Investment Companies [Member]
Investments in securities:
Total investments in securities	107,083	23,481	107,083	23,481	$ 23,861
Trading - Common stocks [Member]
Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis [Roll Forward]
Beginning Balance	939	700	1,293	700
Total Realized and Unrealized Gains or (Losses) - Trading	(22)	16	(21)	16
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)	(22)	16	(21)	16
Purchases	3	0	6	0
Sales	0	0	358	0
Transfers In and/or (Out) of Level 3	0	0	0	0
Ending Balance	920	716	920	716
Trading - Other [Member]
Changes in Level 3 Assets and Liabilities Measured at Fair Value on a Recurring Basis [Roll Forward]
Beginning Balance	279	284	294	284
Total Realized and Unrealized Gains or (Losses) - Trading	21	0	83	0
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)	21	0	83	0
Purchases	0	10	5	10
Sales	(2)	0	84	0
Transfers In and/or (Out) of Level 3	0	0	0	0
Ending Balance	$ 298	$ 294	$ 298	$ 294