Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 89,423	$ 122,875
Financial instruments owned	944,813	786,855
Available-for-sale securities, Estimated fair value	787,897	1,513,478
Investments	190,452	210,255
Total Assets	2,012,585	2,633,463
Financial instruments sold, but not yet purchased, at fair value	566,726	587,265
Derivative contracts, Liabilities	4,322	5,641
Total Liabilities	571,048	592,906
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	89,423	122,875
Financial instruments owned	100,567	210,733
Available-for-sale securities, Estimated fair value		50,892
Investments	66,167	54,654
Total Assets	256,157	439,154
Financial instruments sold, but not yet purchased, at fair value	302,170	269,122
Total Liabilities	302,170	269,122
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	838,385	572,179
Available-for-sale securities, Estimated fair value	787,897	1,418,541
Investments	2,402	35,088
Total Assets	1,628,684	2,025,808
Financial instruments sold, but not yet purchased, at fair value	264,556	318,143
Derivative contracts, Liabilities	4,322	5,641
Total Liabilities	268,878	323,784
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,861	3,943
Available-for-sale securities, Estimated fair value		44,045
Investments	121,883	120,513
Total Assets	127,744	168,501
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	29,280	58,992
Investments	103	6,555
Financial instruments sold, but not yet purchased, at fair value	236,053	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	29,280	58,992
Investments	103	104
Financial instruments sold, but not yet purchased, at fair value	236,053	146,592
U.S. Government Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		6,451
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	151,895	101,439
Available-for-sale securities, Estimated fair value	1,710	1,610
Financial instruments sold, but not yet purchased, at fair value	5,061	10,029
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	151,895	101,439
Available-for-sale securities, Estimated fair value	1,710	1,610
Financial instruments sold, but not yet purchased, at fair value	5,061	10,029
Mortgage-backed Securities: Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	259,968	159,057
Available-for-sale securities, Estimated fair value	27,903	209,206
Financial instruments sold, but not yet purchased, at fair value	70,715	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	259,968	159,057
Available-for-sale securities, Estimated fair value	27,903	209,206
Financial instruments sold, but not yet purchased, at fair value	70,715	28,067
Mortgage-backed Securities: Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	16,377	13,366
Available-for-sale securities, Estimated fair value	2,934	3,137
Financial instruments sold, but not yet purchased, at fair value	163	4,556
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
Financial instruments sold, but not yet purchased, at fair value		401
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	15,707	12,371
Available-for-sale securities, Estimated fair value	2,934	3,137
Financial instruments sold, but not yet purchased, at fair value	163	4,155
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	670	806
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	270,594	245,909
Available-for-sale securities, Estimated fair value	188,533	337,406
Financial instruments sold, but not yet purchased, at fair value	189,758	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	30,389	75,236
Available-for-sale securities, Estimated fair value		50,892
Financial instruments sold, but not yet purchased, at fair value	1,194	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	235,633	168,680
Available-for-sale securities, Estimated fair value	188,533	286,514
Financial instruments sold, but not yet purchased, at fair value	188,564	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	4,572	1,993
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	41,628	77,548
Investments	54,126	59,203
Financial instruments sold, but not yet purchased, at fair value	64,970	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	40,898	76,316
Investments	51,157	35,123
Financial instruments sold, but not yet purchased, at fair value	64,923	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	111	88
Investments	7	24,080
Financial instruments sold, but not yet purchased, at fair value	47
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
Investments	2,962
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	175,071	130,544
Available-for-sale securities, Estimated fair value	73,733	74,401
Financial instruments sold, but not yet purchased, at fair value	6
State And Municipal Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	175,071	130,544
Available-for-sale securities, Estimated fair value	73,733	74,401
Financial instruments sold, but not yet purchased, at fair value	6
Mortgage-backed Securities: Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	36,654	107,644
Mortgage-backed Securities: 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	36,654	107,644
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	456,430	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	456,430	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		44,045
Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	14,907	18,144
Mutual Funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	14,907	18,144
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	48,355	46,197
Auction Rate Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	48,355	46,197
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,324	1,326
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,324	1,326
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	71,637	78,830
Other Investment [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		1,283
Other Investment [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	2,395	4,557
Other Investment [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	69,242	72,990
Investments In Partnerships [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	51,200	42,100
Private Company Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	13,200	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