Fair Value Of Financial Instruments (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	$ 1,148,785	$ 749,443
Available-for-sale securities	3,145,267	1,629,907
Investments	153,595	181,017
Cash equivalents measured at NAV	19,946	77,097
Total Assets	4,467,593	2,637,464
Financial instruments sold, but not yet purchased, at fair value	665,757	521,744
Derivative contracts, Liabilities	10,599	3,591
Total Liabilities	676,356	525,335
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	51,699	87,758
Available-for-sale securities	402
Investments	22,040	26,542
Total Assets	74,141	114,300
Financial instruments sold, but not yet purchased, at fair value	343,611	212,525
Total Liabilities	343,611	212,525
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,095,185	659,590
Available-for-sale securities	3,144,865	1,629,907
Investments	383	8,141
Total Assets	4,240,433	2,297,638
Financial instruments sold, but not yet purchased, at fair value	322,146	309,219
Derivative contracts, Liabilities	10,599	3,591
Total Liabilities	332,745	312,810
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,901	2,095
Investments	60,575	56,078
Total Assets	62,476	58,173
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	226,632	116,949
Available-for-sale securities	4,446	1,698
U.S. Government Agency Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	402
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	226,632	116,949
Available-for-sale securities	4,044	1,698
Corporate Fixed Income Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	271,785	203,910
Available-for-sale securities	828,770	319,408
Financial instruments sold, but not yet purchased, at fair value	207,080	255,700
Corporate Fixed Income Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	5,099	13,203
Financial instruments sold, but not yet purchased, at fair value	1,487	3,601
Corporate Fixed Income Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	266,407	190,707
Available-for-sale securities	828,770	319,408
Financial instruments sold, but not yet purchased, at fair value	205,593	252,099
Corporate Fixed Income Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	279
Corporate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	27,995	31,642
Investments	26,385	30,737
Financial instruments sold, but not yet purchased, at fair value	22,905	29,184
Corporate Equity Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	26,984	29,388
Investments	22,040	26,436
Financial instruments sold, but not yet purchased, at fair value	22,905	22,894
Corporate Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	545	1,635
Financial instruments sold, but not yet purchased, at fair value		6,290
Corporate Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	466	619
Investments	4,345	4,301
State And Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	271,722	112,983
Available-for-sale securities	73,860	74,167
Financial instruments sold, but not yet purchased, at fair value	47
State And Municipal Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	271,722	112,983
Available-for-sale securities	73,860	74,167
Financial instruments sold, but not yet purchased, at fair value	47
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,793,193	915,913
Asset-Backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,793,193	915,913
Auction Rate Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	50,607	55,710
Auction Rate Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		5,268
Auction Rate Equity Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	50,607	50,442
Auction Rate Municipal Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,356	1,315
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	1,356	1,315
Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,650	2,897
Other Investment [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments		4
Other Investment [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	383	2,873
Other Investment [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	4,267	20
Investments In Funds Measured At NAV [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments	70,597	90,256
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,616	45,167
Investments		102
Financial instruments sold, but not yet purchased, at fair value	319,219	186,030
U.S. Government Securities [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	19,616	45,167
Investments		102
Financial instruments sold, but not yet purchased, at fair value	319,219	186,030
Mortgage Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	295,666	205,473
Available-for-sale securities	365,636	304,893
Financial instruments sold, but not yet purchased, at fair value	116,506	50,830
Mortgage Backed Securities [Member] | Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	295,666	205,473
Available-for-sale securities	365,636	304,893
Financial instruments sold, but not yet purchased, at fair value	116,506	50,830
Mortgage Backed Securities [Member] | Non-Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	35,369	33,319
Available-for-sale securities	2,091	2,518
Mortgage Backed Securities [Member] | Non-Agency [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	34,213	31,843
Available-for-sale securities	2,091	2,518
Mortgage Backed Securities [Member] | Non-Agency [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial instruments owned	1,156	1,476
Mortgage Backed Securities [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	77,271	11,310
Mortgage Backed Securities [Member] | Commercial [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 77,271	$ 11,310