Fair Value Of Financial Instruments (Schedule Of Valuation Of Financial Instruments By Pricing Observability Levels) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Cash and cash equivalents	$ 152,798		$ 72,596
Trading securities owned, at fair value	741,420		763,608
Available-for-sale securities, Estimated fair value	2,260,005		1,625,168
Investments	255,202		236,434
Total Assets	3,409,425		2,697,806
Trading securities sold, but not yet purchased, at fair value	480,907		319,742
Derivative contracts, Liabilities	11,884	11,884	19,934
Total Liabilities	518,206		362,642
Level 1 [Member]
Cash and cash equivalents	152,798		72,596
Trading securities owned, at fair value	177,779		104,018
Available-for-sale securities, Estimated fair value	358,884		263,017
Investments	87,941		58,872
Total Assets	777,402		498,503
Trading securities sold, but not yet purchased, at fair value	316,997		214,871
Total Liabilities	342,412		237,837
Level 2 [Member]
Trading securities owned, at fair value	545,983		659,007
Available-for-sale securities, Estimated fair value	1,819,191		1,271,664
Investments	4,828		61,801
Total Assets	2,370,002		1,992,472
Trading securities sold, but not yet purchased, at fair value	163,910		104,871
Derivative contracts, Liabilities	11,884		19,934
Total Liabilities	175,794		124,805
Level 3 [Member]
Trading securities owned, at fair value	17,658		583
Available-for-sale securities, Estimated fair value	81,930		90,487
Investments	162,433		115,761
Total Assets	262,021		206,831
U.S. Government Agency Securities [Member]
Trading securities owned, at fair value	154,241		123,758
Available-for-sale securities, Estimated fair value	1,212		1,113
Trading securities sold, but not yet purchased, at fair value	9,599		15
U.S. Government Agency Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	154,241		123,758
Available-for-sale securities, Estimated fair value	1,212		1,113
Trading securities sold, but not yet purchased, at fair value	9,599		15
U.S. Government Securities [Member]
Trading securities owned, at fair value	14,695		3,573
Investments			7,069
Trading securities sold, but not yet purchased, at fair value	148,888		162,661
U.S. Government Securities [Member] | Level 1 [Member]
Trading securities owned, at fair value	14,695		3,573
Investments			7,069
Trading securities sold, but not yet purchased, at fair value	148,888		162,661
Corporate Fixed Income Securities [Member]
Trading securities owned, at fair value	358,447		396,878
Available-for-sale securities, Estimated fair value	526,777		480,182
Trading securities sold, but not yet purchased, at fair value	230,046		150,698
Corporate Fixed Income Securities [Member] | Level 1 [Member]
Trading securities owned, at fair value	66,707		66,795
Available-for-sale securities, Estimated fair value	358,884		263,017
Trading securities sold, but not yet purchased, at fair value	76,439		46,274
Corporate Fixed Income Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	286,683		329,500
Available-for-sale securities, Estimated fair value	167,893		217,165
Trading securities sold, but not yet purchased, at fair value	153,607		104,424
Corporate Fixed Income Securities [Member] | Level 3 [Member]
Trading securities owned, at fair value	5,057		583
Corporate Equity Securities [Member]
Trading securities owned, at fair value	111,549		35,472
Investments	67,858		32,162
Trading securities sold, but not yet purchased, at fair value	92,157		6,281
Corporate Equity Securities [Member] | Level 1 [Member]
Trading securities owned, at fair value	96,377		33,650
Investments	67,858		32,162
Trading securities sold, but not yet purchased, at fair value	91,670		5,936
Corporate Equity Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	2,571		1,822
Trading securities sold, but not yet purchased, at fair value	487		345
Corporate Equity Securities [Member] | Level 3 [Member]
Trading securities owned, at fair value	12,601
Corporate Preferred Securities [Member]
Investments			56,970
Corporate Preferred Securities [Member] | Level 2 [Member]
Investments			56,970
State And Municipal Securities [Member]
Trading securities owned, at fair value	102,488		203,927
Available-for-sale securities, Estimated fair value	168,868		157,420
Trading securities sold, but not yet purchased, at fair value	217		87
State And Municipal Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	102,488		203,927
Available-for-sale securities, Estimated fair value	86,938		66,933
Trading securities sold, but not yet purchased, at fair value	217		87
State And Municipal Securities [Member] | Level 3 [Member]
Available-for-sale securities, Estimated fair value	81,930		90,487
Agency [Member]
Available-for-sale securities, Estimated fair value	1,230,661		684,848
Agency [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	1,230,661		684,848
Commercial [Member]
Available-for-sale securities, Estimated fair value	207,857		260,974
Commercial [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	207,857		260,974
Non-Agency [Member]
Available-for-sale securities, Estimated fair value	5,741		13,878
Non-Agency [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	5,741		13,878
Asset-Backed Securities [Member]
Available-for-sale securities, Estimated fair value	118,889		26,753
Asset-Backed Securities [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	118,889		26,753
Mutual Funds [Member]
Investments	19,561		18,021
Mutual Funds [Member] | Level 1 [Member]
Investments	19,561		18,021
Securities Sold, But Not yet Purchased [Member]
Trading securities sold, but not yet purchased, at fair value	25,415		22,966
Securities Sold, But Not yet Purchased [Member] | Level 1 [Member]
Trading securities sold, but not yet purchased, at fair value	25,415		22,966
Auction Rate Equity Securities [Member]
Investments	62,109		64,397
Auction Rate Equity Securities [Member] | Level 3 [Member]
Investments	62,109		64,397
Auction Rate Municipal Securities [Member]
Investments	13,330		14,067
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Investments	13,330		14,067
Other Investment [Member]
Investments	92,344		43,748
Other Investment [Member] | Level 1 [Member]
Investments	522		1,620
Other Investment [Member] | Level 2 [Member]
Investments	4,828		4,831
Other Investment [Member] | Level 3 [Member]
Investments	$ 86,994		$ 37,297