Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 91,268	$ 122,875
Financial instruments owned, at fair value	749,443	786,855
Available-for-sale securities, at fair value	1,629,907	1,513,478
Investments	181,017	210,255
Total Assets	2,651,635	2,633,463
Financial instruments sold, but not yet purchased, at fair value	521,744	587,265
Derivative contracts, Liabilities	3,591	5,641
Total Liabilities	525,335	592,906
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	45,167	58,992
Investments	102	6,555
Financial instruments sold, but not yet purchased, at fair value	186,030	146,592
U S Government Corporations And Agencies Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	116,949	101,439
Available-for-sale securities, at fair value	1,698	1,610
Financial instruments sold, but not yet purchased, at fair value		10,029
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	203,910	245,909
Available-for-sale securities, at fair value	319,408	337,406
Financial instruments sold, but not yet purchased, at fair value	255,700	293,008
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	31,642	77,548
Investments	30,737	59,203
Financial instruments sold, but not yet purchased, at fair value	29,184	105,013
U S States And Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	112,983	130,544
Available-for-sale securities, at fair value	74,167	74,401
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	915,913	780,074
Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	15,493	18,144
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	55,710	46,197
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,315	1,326
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	77,660	78,830
Partnership Interest
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	54,500	42,100
Private Company Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	14,600	16,400
Private Equity And Other Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	2,800	14,300
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	91,268	122,875
Financial instruments owned, at fair value	87,758	210,733
Available-for-sale securities, at fair value		50,892
Investments	46,942	54,654
Total Assets	225,968	439,154
Financial instruments sold, but not yet purchased, at fair value	212,525	269,122
Total Liabilities	212,525	269,122
Level 1 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	45,167	58,992
Investments	102	104
Financial instruments sold, but not yet purchased, at fair value	186,030	146,592
Level 1 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	13,203	75,236
Available-for-sale securities, at fair value		50,892
Financial instruments sold, but not yet purchased, at fair value	3,601	17,116
Level 1 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	29,388	76,316
Investments	26,436	35,123
Financial instruments sold, but not yet purchased, at fair value	22,894	105,013
Level 1 [Member] | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	15,493	18,144
Level 1 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,911	1,283
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	659,590	572,179
Available-for-sale securities, at fair value	1,629,907	1,418,541
Investments	8,141	35,088
Total Assets	2,297,638	2,025,808
Financial instruments sold, but not yet purchased, at fair value	309,219	318,143
Derivative contracts, Liabilities	3,591	5,641
Total Liabilities	312,810	323,784
Level 2 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		6,451
Level 2 [Member] | U S Government Corporations And Agencies Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	116,949	101,439
Available-for-sale securities, at fair value	1,698	1,610
Financial instruments sold, but not yet purchased, at fair value		10,029
Level 2 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	190,707	168,680
Available-for-sale securities, at fair value	319,408	286,514
Financial instruments sold, but not yet purchased, at fair value	252,099	275,892
Level 2 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	1,635	88
Investments		24,080
Financial instruments sold, but not yet purchased, at fair value	6,290
Level 2 [Member] | U S States And Political Subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	112,983	130,544
Available-for-sale securities, at fair value	74,167	74,401
Level 2 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	915,913	736,029
Level 2 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	5,268
Level 2 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	2,873	4,557
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	2,095	3,943
Available-for-sale securities, at fair value		44,045
Investments	125,934	120,513
Total Assets	128,029	168,501
Level 3 [Member] | Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value		1,993
Level 3 [Member] | Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	619	1,144
Investments	4,301
Level 3 [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value		44,045
Level 3 [Member] | Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	50,442	46,197
Level 3 [Member] | Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,315	1,326
Level 3 [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	69,876	72,990
Mortgage Backed Securities | Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	205,473	159,057
Available-for-sale securities, at fair value	304,893	209,206
Financial instruments sold, but not yet purchased, at fair value	50,830	28,067
Mortgage Backed Securities | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	33,319	13,366
Available-for-sale securities, at fair value	2,518	3,137
Financial instruments sold, but not yet purchased, at fair value		4,556
Mortgage Backed Securities | Commercial Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	11,310	107,644
Mortgage Backed Securities | Level 1 [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value		189
Financial instruments sold, but not yet purchased, at fair value		401
Mortgage Backed Securities | Level 2 [Member] | Agency Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	205,473	159,057
Available-for-sale securities, at fair value	304,893	209,206
Financial instruments sold, but not yet purchased, at fair value	50,830	28,067
Mortgage Backed Securities | Level 2 [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	31,843	12,371
Available-for-sale securities, at fair value	2,518	3,137
Financial instruments sold, but not yet purchased, at fair value		4,155
Mortgage Backed Securities | Level 2 [Member] | Commercial Mortgage Backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	11,310	107,644
Mortgage Backed Securities | Level 3 [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned, at fair value	$ 1,476	$ 806