Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,065,503	$ 925,045
Available-for-sale securities	3,694,298	3,181,313
Investments	112,649	133,563
Cash equivalents measured at NAV	16,528	35,637
Derivative contracts, Assets	5,669	10,390
Total Assets	4,894,647	4,285,948
Financial instruments sold, but not yet purchased, at fair value	749,323	699,032
Derivative contracts, Liabilities		1,823
Total Liabilities		700,855
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	128,014	89,833
Available-for-sale securities	4,686	4,197
Financial instruments sold, but not yet purchased, at fair value	22,140	20,549
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	409,550	299,946
Available-for-sale securities	1,072,376	823,511
Financial instruments sold, but not yet purchased, at fair value	311,516	202,968
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	38,280	32,044
Investments	51,475	27,247
Financial instruments sold, but not yet purchased, at fair value	52,261	18,395
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	145,226	159,095
Available-for-sale securities	71,961	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	2,159,035	1,867,718
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	34,608	48,689
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	840	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	24,116	55,172
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	48,900	9,951
Financial instruments sold, but not yet purchased, at fair value	297,690	362,536
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	258,348	305,774
Available-for-sale securities	311,888	338,732
Financial instruments sold, but not yet purchased, at fair value	65,716	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	37,185	28,402
Available-for-sale securities	1,619	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	72,733	72,773
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	95,412	43,339
Available-for-sale securities	199	300
Investments	51,235	23,414
Total Assets	146,846	67,053
Financial instruments sold, but not yet purchased, at fair value	383,744	381,911
Total Liabilities		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	8,499	1,944
Financial instruments sold, but not yet purchased, at fair value	33,793	980
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	38,013	31,444
Investments	51,235	23,414
Financial instruments sold, but not yet purchased, at fair value	52,261	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	48,900	9,951
Financial instruments sold, but not yet purchased, at fair value	297,690	362,536
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	969,218	879,751
Available-for-sale securities	3,694,099	3,181,013
Investments	370	383
Derivative contracts, Assets	5,669	10,390
Total Assets	4,669,356	4,071,537
Financial instruments sold, but not yet purchased, at fair value	365,579	317,121
Derivative contracts, Liabilities		1,823
Total Liabilities		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	128,014	89,833
Available-for-sale securities	4,487	3,897
Financial instruments sold, but not yet purchased, at fair value	22,140	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	400,803	297,729
Available-for-sale securities	1,072,376	823,511
Financial instruments sold, but not yet purchased, at fair value	277,723	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	145,226	159,095
Available-for-sale securities	71,961	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	2,159,035	1,867,718
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	258,348	305,774
Available-for-sale securities	311,888	338,732
Financial instruments sold, but not yet purchased, at fair value	65,716	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	36,827	27,320
Available-for-sale securities	1,619	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	72,733	72,773
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	873	1,955
Investments	36,928	54,594
Total Assets	37,801	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	248	273
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	267	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	34,608	48,689
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	840	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	$ 358	$ 1,082