Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,058,089	$ 925,045
Available-for-sale securities	3,377,981	3,181,313
Investments	133,761	133,563
Cash equivalents measured at NAV	15,420	35,637
Derivative contracts, Assets	11,300	10,390
Total Assets	4,596,551	4,285,948
Financial instruments sold, but not yet purchased, at fair value	754,458	699,032
Derivative contracts, Liabilities	1,191	1,823
Total Liabilities	755,649	700,855
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	170,659	89,833
Available-for-sale securities	4,419	4,197
Financial instruments sold, but not yet purchased, at fair value		20,549
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	356,364	299,946
Available-for-sale securities	824,192	823,511
Financial instruments sold, but not yet purchased, at fair value	260,410	202,968
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	39,791	32,044
Investments	27,866	27,247
Financial instruments sold, but not yet purchased, at fair value	45,308	18,395
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	149,242	159,095
Available-for-sale securities	72,417	72,490
Financial instruments sold, but not yet purchased, at fair value		31
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,061,410	1,867,718
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	49,055	48,689
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	837	832
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,620	1,623
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	54,383	55,172
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,784	9,951
Financial instruments sold, but not yet purchased, at fair value	351,765	362,536
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	293,654	305,774
Available-for-sale securities	341,054	338,732
Financial instruments sold, but not yet purchased, at fair value	96,975	94,552
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	28,595	28,402
Available-for-sale securities	1,731	1,892
Financial instruments sold, but not yet purchased, at fair value		1
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	72,758	72,773
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	67,987	43,339
Available-for-sale securities	199	300
Investments	24,033	23,414
Total Assets	92,219	67,053
Financial instruments sold, but not yet purchased, at fair value	397,073	381,911
Total Liabilities	397,073	381,911
Level 1 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	199	300
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	8,995	1,944
Financial instruments sold, but not yet purchased, at fair value		980
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	39,208	31,444
Investments	24,033	23,414
Financial instruments sold, but not yet purchased, at fair value	45,308	18,395
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,784	9,951
Financial instruments sold, but not yet purchased, at fair value	351,765	362,536
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	988,233	879,751
Available-for-sale securities	3,377,782	3,181,013
Investments	380	383
Derivative contracts, Assets	11,300	10,390
Total Assets	4,377,695	4,071,537
Financial instruments sold, but not yet purchased, at fair value	357,385	317,121
Derivative contracts, Liabilities	1,191	1,823
Total Liabilities	358,576	318,944
Level 2 [Member] | U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	170,659	89,833
Available-for-sale securities	4,220	3,897
Financial instruments sold, but not yet purchased, at fair value		20,549
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	347,102	297,729
Available-for-sale securities	824,192	823,511
Financial instruments sold, but not yet purchased, at fair value	260,410	201,988
Level 2 [Member] | State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	149,242	159,095
Available-for-sale securities	72,417	72,490
Financial instruments sold, but not yet purchased, at fair value		31
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,061,410	1,867,718
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	380	383
Level 2 [Member] | Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	293,654	305,774
Available-for-sale securities	341,054	338,732
Financial instruments sold, but not yet purchased, at fair value	96,975	94,552
Level 2 [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27,576	27,320
Available-for-sale securities	1,731	1,892
Financial instruments sold, but not yet purchased, at fair value		1
Level 2 [Member] | Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	72,758	72,773
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,869	1,955
Investments	54,965	54,594
Total Assets	56,834	56,549
Level 3 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	267	273
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	583	600
Investments	3,833	3,833
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	49,055	48,689
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	837	832
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,240	1,240
Level 3 [Member] | Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,019	$ 1,082