Fair Value Of Financial Instruments (Schedule Of Valuation Of Financial Instruments By Pricing Observability Levels) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Cash and cash equivalents	$ 138,833	$ 72,596
Trading securities owned, at fair value	847,652	763,608
Available-for-sale securities, Estimated fair value	1,231,571	1,625,168
Investments	233,778	236,434
Total Assets	2,451,834	2,697,806
Trading securities sold, but not yet purchased, at fair value	495,658	319,742
Derivative contracts, Liabilities	11,101	19,934
Total Liabilities	535,357	362,642
Level 1 [Member]
Cash and cash equivalents	138,833	72,596
Trading securities owned, at fair value	144,409	104,018
Available-for-sale securities, Estimated fair value	62,743	263,017
Investments	55,898	58,872
Total Assets	401,883	498,503
Trading securities sold, but not yet purchased, at fair value	264,262	214,871
Total Liabilities	292,860	237,837
Level 2 [Member]
Trading securities owned, at fair value	702,446	659,007
Available-for-sale securities, Estimated fair value	1,090,393	1,271,664
Investments	4,837	61,801
Total Assets	1,797,676	1,992,472
Trading securities sold, but not yet purchased, at fair value	231,396	104,871
Derivative contracts, Liabilities	11,101	19,934
Total Liabilities	242,497	124,805
Level 3 [Member]
Trading securities owned, at fair value	797	583
Available-for-sale securities, Estimated fair value	78,435	90,487
Investments	173,043	115,761
Total Assets	252,275	206,831
U.S. Government Agency Securities [Member]
Trading securities owned, at fair value	121,136	123,758
Available-for-sale securities, Estimated fair value	1,226	1,113
Trading securities sold, but not yet purchased, at fair value	23,248	15
U.S. Government Agency Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	121,136	123,758
Available-for-sale securities, Estimated fair value	1,226	1,113
Trading securities sold, but not yet purchased, at fair value	23,248	15
U.S. Government Securities [Member]
Trading securities owned, at fair value	13,627	3,573
Investments		7,069
Trading securities sold, but not yet purchased, at fair value	174,688	162,661
U.S. Government Securities [Member] | Level 1 [Member]
Trading securities owned, at fair value	13,627	3,573
Investments		7,069
Trading securities sold, but not yet purchased, at fair value	174,688	162,661
Corporate Fixed Income Securities [Member]
Trading securities owned, at fair value	519,180	396,878
Available-for-sale securities, Estimated fair value	499,966	480,182
Trading securities sold, but not yet purchased, at fair value	213,757	150,698
Corporate Fixed Income Securities [Member] | Level 1 [Member]
Trading securities owned, at fair value	46,644	66,795
Available-for-sale securities, Estimated fair value	62,743	263,017
Trading securities sold, but not yet purchased, at fair value	6,150	46,274
Corporate Fixed Income Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	471,739	329,500
Available-for-sale securities, Estimated fair value	437,223	217,165
Trading securities sold, but not yet purchased, at fair value	207,607	104,424
Corporate Fixed Income Securities [Member] | Level 3 [Member]
Trading securities owned, at fair value	797	583
Corporate Equity Securities [Member]
Trading securities owned, at fair value	85,183	35,472
Investments	39,212	32,162
Trading securities sold, but not yet purchased, at fair value	83,881	6,281
Corporate Equity Securities [Member] | Level 1 [Member]
Trading securities owned, at fair value	84,138	33,650
Investments	39,212	32,162
Trading securities sold, but not yet purchased, at fair value	83,424	5,936
Corporate Equity Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	1,045	1,822
Trading securities sold, but not yet purchased, at fair value	457	345
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	108,526	203,927
Available-for-sale securities, Estimated fair value	163,414	157,420
Trading securities sold, but not yet purchased, at fair value	84	87
State And Municipal Securities [Member] | Level 2 [Member]
Trading securities owned, at fair value	108,526	203,927
Available-for-sale securities, Estimated fair value	84,979	66,933
Trading securities sold, but not yet purchased, at fair value	84	87
State And Municipal Securities [Member] | Level 3 [Member]
Available-for-sale securities, Estimated fair value	78,435	90,487
Agency [Member]
Available-for-sale securities, Estimated fair value	176,933	684,848
Agency [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	176,933	684,848
Commercial [Member]
Available-for-sale securities, Estimated fair value	231,709	260,974
Commercial [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	231,709	260,974
Non-Agency [Member]
Available-for-sale securities, Estimated fair value	4,838	13,878
Non-Agency [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	4,838	13,878
Asset-Backed Securities [Member]
Available-for-sale securities, Estimated fair value	153,485	26,753
Asset-Backed Securities [Member] | Level 2 [Member]
Available-for-sale securities, Estimated fair value	153,485	26,753
Mutual Funds [Member]
Investments	15,805	18,021
Mutual Funds [Member] | Level 1 [Member]
Investments	15,805	18,021
Securities Sold, But Not yet Purchased [Member]
Trading securities sold, but not yet purchased, at fair value	28,598	22,966
Securities Sold, But Not yet Purchased [Member] | Level 1 [Member]
Trading securities sold, but not yet purchased, at fair value	28,598	22,966
Auction Rate Equity Securities [Member]
Investments	59,728	64,397
Auction Rate Equity Securities [Member] | Level 3 [Member]
Investments	59,728	64,397
Auction Rate Municipal Securities [Member]
Investments	12,409	14,067
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Investments	12,409	14,067
Other Investment [Member]
Investments	106,624	43,748
Other Investment [Member] | Level 1 [Member]
Investments	881	1,620
Other Investment [Member] | Level 2 [Member]
Investments	4,837	4,831
Other Investment [Member] | Level 3 [Member]
Investments	$ 100,906	$ 37,297