Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 85,576	$ 122,875
Financial instruments owned	812,232	786,855
Available-for-sale securities, Estimated fair value	1,451,260	1,513,478
Investments	198,260	210,255
Total Assets	2,547,328	2,633,463
Financial instruments sold, but not yet purchased, at fair value	544,881	587,265
Derivative contracts, Liabilities	5,932	5,641
Total Liabilities	550,813	592,906
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	85,576	122,875
Financial instruments owned	168,316	210,733
Available-for-sale securities, Estimated fair value	55,415	50,892
Investments	72,028	54,654
Total Assets	381,335	439,154
Financial instruments sold, but not yet purchased, at fair value	321,026	269,122
Total Liabilities	321,026	269,122
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	639,898	572,179
Available-for-sale securities, Estimated fair value	1,351,458	1,418,541
Investments	15,827	35,088
Total Assets	2,007,183	2,025,808
Financial instruments sold, but not yet purchased, at fair value	223,855	318,143
Derivative contracts, Liabilities	5,932	5,641
Total Liabilities	229,787	323,784
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	4,018	3,943
Available-for-sale securities, Estimated fair value	44,387	44,045
Investments	110,405	120,513
Total Assets	158,810	168,501
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	96,319	58,992
Available-for-sale securities, Estimated fair value	1,712	1,610
Financial instruments sold, but not yet purchased, at fair value	4,965	10,029
U.S. Government Agency Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		58,992
U.S. Government Agency Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	96,319
Available-for-sale securities, Estimated fair value	1,712	1,610
Financial instruments sold, but not yet purchased, at fair value	4,965	10,029
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	39,200	101,439
Investments	13,518	6,555
Financial instruments sold, but not yet purchased, at fair value	227,211	146,592
U.S. Government Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	39,200
Investments	104	104
Financial instruments sold, but not yet purchased, at fair value	227,211	146,592
U.S. Government Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		101,439
Investments	13,414	6,451
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	184,591	245,909
Available-for-sale securities, Estimated fair value	305,352	337,406
Financial instruments sold, but not yet purchased, at fair value	159,976	293,008
Corporate Fixed Income Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	48,819	75,236
Available-for-sale securities, Estimated fair value	55,415	50,892
Financial instruments sold, but not yet purchased, at fair value	4,238	17,116
Corporate Fixed Income Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	133,092	168,680
Available-for-sale securities, Estimated fair value	249,937	286,514
Financial instruments sold, but not yet purchased, at fair value	155,738	275,892
Corporate Fixed Income Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	2,680	1,993
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	89,982	77,548
Investments	52,222	59,203
Financial instruments sold, but not yet purchased, at fair value	108,114	105,013
Corporate Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	80,297	76,316
Investments	52,215	35,123
Financial instruments sold, but not yet purchased, at fair value	89,577	105,013
Corporate Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	9,066	88
Investments	7	24,080
Financial instruments sold, but not yet purchased, at fair value	18,537
Corporate Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	619	1,144
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	188,726	130,544
Available-for-sale securities, Estimated fair value	75,247	74,401
Financial instruments sold, but not yet purchased, at fair value	10
State And Municipal Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	188,726	130,544
Available-for-sale securities, Estimated fair value	75,247	74,401
Financial instruments sold, but not yet purchased, at fair value	10
Mortgage-backed Securities: Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	184,796	159,057
Available-for-sale securities, Estimated fair value	199,640	209,206
Financial instruments sold, but not yet purchased, at fair value	44,605	28,067
Mortgage-backed Securities: Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	184,796	159,057
Available-for-sale securities, Estimated fair value	199,640	209,206
Financial instruments sold, but not yet purchased, at fair value	44,605	28,067
Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	93,784	107,644
Financial instruments sold, but not yet purchased, at fair value		4,556
Commercial [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, but not yet purchased, at fair value		401
Commercial [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	93,784	107,644
Financial instruments sold, but not yet purchased, at fair value		4,155
Mortgage-backed Securities: Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	28,618	13,366
Available-for-sale securities, Estimated fair value	3,039	3,137
Mortgage-backed Securities: Non-Agency [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned		189
Mortgage-backed Securities: Non-Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27,899	12,371
Available-for-sale securities, Estimated fair value	3,039	3,137
Mortgage-backed Securities: Non-Agency [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	719	806
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	772,486	780,074
Asset-Backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	728,099	736,029
Asset-Backed Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	44,387	44,045
Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	14,860	18,144
Mutual Funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	14,860	18,144
Securities Sold, But Not yet Purchased [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities sold, but not yet purchased, at fair value
Securities Sold, But Not yet Purchased [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities sold, but not yet purchased, at fair value
Securities Sold, But Not yet Purchased [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities sold, but not yet purchased, at fair value
Securities Sold, But Not yet Purchased [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities sold, but not yet purchased, at fair value
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	42,518	46,197
Auction Rate Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	42,518	46,197
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,330	1,326
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,330	1,326
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	73,812	78,830
Other Investment [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,849	1,283
Other Investment [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	2,406	4,557
Other Investment [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	66,557	72,990
Investments In Partnerships [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	48,400	42,100
Private Company Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	13,300	16,400
Private Equity And Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	$ 4,800	$ 14,300