Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,063,651	$ 925,045
Available-for-sale securities	3,455,373	3,181,313
Investments	128,332	133,563
Cash equivalents measured at NAV	16,365	35,637
Derivative contracts, Assets	8,442	10,390
Total Assets	4,672,163	4,285,948
Financial instruments sold, but not yet purchased, at fair value	705,577	699,032
Derivative contracts, Liabilities	939	1,823
Total Liabilities	706,516	700,855
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	119,703	89,833
Available-for-sale securities	4,565	4,197
Financial instruments sold, but not yet purchased, at fair value	1,995	20,549
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	367,866	299,946
Available-for-sale securities	971,131	823,511
Financial instruments sold, but not yet purchased, at fair value	245,148	202,968
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	43,671	32,044
Investments	50,384	27,247
Financial instruments sold, but not yet purchased, at fair value	41,091	18,395
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	147,458	159,095
Available-for-sale securities	72,382	72,490
Financial instruments sold, but not yet purchased, at fair value	37	31
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	2,013,928	1,867,718
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	48,577	48,689
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	841	832
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,610	1,623
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	26,920	55,172
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	16,924	9,951
Financial instruments sold, but not yet purchased, at fair value	298,832	362,536
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	313,463	305,774
Available-for-sale securities	319,062	338,732
Financial instruments sold, but not yet purchased, at fair value	118,458	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	54,566	28,402
Available-for-sale securities	1,617	1,892
Financial instruments sold, but not yet purchased, at fair value	16	1
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	72,688	72,773
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	67,671	43,339
Available-for-sale securities	199	300
Investments	50,144	23,414
Total Assets	118,014	67,053
Financial instruments sold, but not yet purchased, at fair value	340,240	381,911
Total Liabilities	340,240	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	7,443	1,944
Financial instruments sold, but not yet purchased, at fair value	317	980
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	43,304	31,444
Investments	50,144	23,414
Financial instruments sold, but not yet purchased, at fair value	41,091	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	16,924	9,951
Financial instruments sold, but not yet purchased, at fair value	298,832	362,536
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	994,832	879,751
Available-for-sale securities	3,455,174	3,181,013
Investments	14,326	383
Derivative contracts, Assets	8,442	10,390
Total Assets	4,472,774	4,071,537
Financial instruments sold, but not yet purchased, at fair value	365,337	317,121
Derivative contracts, Liabilities	939	1,823
Total Liabilities	366,276	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	119,703	89,833
Available-for-sale securities	4,366	3,897
Financial instruments sold, but not yet purchased, at fair value	1,995	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	360,167	297,729
Available-for-sale securities	971,131	823,511
Financial instruments sold, but not yet purchased, at fair value	244,831	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	147,458	159,095
Available-for-sale securities	72,382	72,490
Financial instruments sold, but not yet purchased, at fair value	37	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	2,013,928	1,867,718
Level 2 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	13,956
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	370	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	313,463	305,774
Available-for-sale securities	319,062	338,732
Financial instruments sold, but not yet purchased, at fair value	118,458	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	54,041	27,320
Available-for-sale securities	1,617	1,892
Financial instruments sold, but not yet purchased, at fair value	16	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	72,688	72,773
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,148	1,955
Investments	36,942	54,594
Total Assets	38,090	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	256	273
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	367	600
Investments	240	3,833
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	34,621	48,689
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	841	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	$ 525	$ 1,082