Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	$ 122,875	$ 78,163
Financial instruments owned	786,855	801,494
Available-for-sale securities, Estimated fair value	1,513,478	1,756,253
Investments	210,255	217,228
Total Assets	2,633,463	2,853,138
Financial instruments sold, but not yet purchased, at fair value	587,265	481,214
Derivative contracts, Liabilities	5,641	9,349
Total Liabilities	592,906	490,563
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	122,875	78,163
Financial instruments owned	210,733	110,469
Available-for-sale securities, Estimated fair value	50,892	83,655
Investments	54,654	49,406
Total Assets	439,154	321,693
Financial instruments sold, but not yet purchased, at fair value	269,122
Total Liabilities	269,122	358,011
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	572,179	688,745
Available-for-sale securities, Estimated fair value	1,418,541	1,608,320
Investments	35,088	2,422
Total Assets	2,025,808	2,299,487
Financial instruments sold, but not yet purchased, at fair value	318,143
Derivative contracts, Liabilities	5,641	9,349
Total Liabilities	323,784	132,552
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	3,943	2,280
Available-for-sale securities, Estimated fair value	44,045	64,278
Investments	120,513	165,400
Total Assets	168,501	231,958
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	58,992	80,835
Available-for-sale securities, Estimated fair value	1,610	1,072
Financial instruments sold, but not yet purchased, at fair value	10,029	253,221
U.S. Government Agency Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	58,992	307
Financial instruments sold, but not yet purchased, at fair value		253,221
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		80,528
Available-for-sale securities, Estimated fair value	1,610	1,072
Financial instruments sold, but not yet purchased, at fair value	10,029
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	101,439	10,594
Investments	6,555
Financial instruments sold, but not yet purchased, at fair value	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		10,594
Investments	104
Financial instruments sold, but not yet purchased, at fair value	146,592
U.S. Government Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	101,439
Investments	6,451
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	59,203	32,402
Financial instruments sold, but not yet purchased, at fair value	105,013
Corporate Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	35,123	32,402
Financial instruments sold, but not yet purchased, at fair value	105,013
Corporate Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	24,080
Mortgage-backed Securities: Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	209,206	183,987
Mortgage-backed Securities: Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	209,206
Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	159,057	267,933
Available-for-sale securities, Estimated fair value		183,987
Financial instruments sold, but not yet purchased, at fair value	28,067	37,201
Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	159,057	267,933
Available-for-sale securities, Estimated fair value		183,987
Financial instruments sold, but not yet purchased, at fair value	28,067	37,201
Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	107,644	211,246
Financial instruments sold, but not yet purchased, at fair value	4,556
Commercial [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, but not yet purchased, at fair value	401
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	107,644	211,246
Financial instruments sold, but not yet purchased, at fair value	4,155
Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	13,366	22,570
Available-for-sale securities, Estimated fair value	3,137	4,619
Non-Agency [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	189
Non-Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	12,371	22,570
Available-for-sale securities, Estimated fair value	3,137	4,619
Non-Agency [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	806
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	245,909	201,579
Available-for-sale securities, Estimated fair value	337,406	498,316
Financial instruments sold, but not yet purchased, at fair value	293,008	100,745
Corporate Fixed Income Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	75,236	23,106
Available-for-sale securities, Estimated fair value	50,892	83,655
Financial instruments sold, but not yet purchased, at fair value	17,116	17,857
Corporate Fixed Income Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	168,680	176,434
Available-for-sale securities, Estimated fair value	286,514	414,661
Financial instruments sold, but not yet purchased, at fair value	275,892	82,888
Corporate Fixed Income Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,993	2,039
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	77,548	76,709
Investments	46,197	56,693
Financial instruments sold, but not yet purchased, at fair value		90,015
Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	76,316	76,462
Financial instruments sold, but not yet purchased, at fair value		86,933
Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	88	6
Financial instruments sold, but not yet purchased, at fair value		3,082
Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,144	241
Investments	46,197	56,693
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	130,544	141,274
Available-for-sale securities, Estimated fair value	74,401	90,677
Financial instruments sold, but not yet purchased, at fair value		32
State And Municipal Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	130,544	141,274
Available-for-sale securities, Estimated fair value	74,401	84,477
Financial instruments sold, but not yet purchased, at fair value		32
State And Municipal 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		6,200
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, Estimated fair value	780,074	766,336
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	736,029	708,258
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	58,078
Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	18,144	16,994
Mutual Funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	18,144	16,994
Securities Sold, But Not yet Purchased [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, but not yet purchased, at fair value		481,214
Securities Sold, But Not yet Purchased [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, but not yet purchased, at fair value		358,011
Securities Sold, But Not yet Purchased [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments sold, but not yet purchased, at fair value		123,203
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,326	10,939
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,326	10,939
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	78,830	100,200
Other Investment [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,283	10
Other Investment [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,557	2,422
Other Investment [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	72,990	97,768
Partnership Interests [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	42,100
Financial instruments sold, but not yet purchased, at fair value	56,000
Private Company Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	16,400
Financial instruments sold, but not yet purchased, at fair value	22,500
Private Equity And Other Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	14,300
Financial instruments sold, but not yet purchased, at fair value	$ 19,200