Fair Value Of Financial Instruments (Schedule Of Valuation Of Financial Instruments By Pricing Observability Levels) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Cash and cash equivalents	$ 72,596	$ 14,156
Trading securities owned	763,608	474,951
Available-for-sale securities	1,625,168	1,214,141
Investments	236,434	211,208
Total Assets	2,697,806	1,914,456
Trading securities sold, but not yet purchased	319,742	266,833
Derivative contracts, Liabilities	19,934	24,877
Total Liabilities	362,642	310,933
Level 1 [Member]
Cash and cash equivalents	72,596	14,156
Trading securities owned	104,018	83,749
Available-for-sale securities	263,017	153,855
Investments	58,872	58,934
Total Assets	498,503	310,694
Trading securities sold, but not yet purchased	214,871	190,514
Total Liabilities	237,837	209,737
Level 2 [Member]
Trading securities owned	659,007	387,460
Available-for-sale securities	1,271,664	981,390
Investments	61,801	336
Total Assets	1,992,472	1,369,186
Trading securities sold, but not yet purchased	104,871	76,319
Derivative contracts, Liabilities	19,934	24,877
Total Liabilities	124,805	101,196
Level 3 [Member]
Trading securities owned	583	3,742
Available-for-sale securities	90,487	78,896
Investments	115,761	151,938
Total Assets	206,831	234,576
U.S. Government Agency Securities [Member]
Trading securities owned	123,758	66,424
Available-for-sale securities	1,113	1,103
Trading securities sold, but not yet purchased	15	954
U.S. Government Agency Securities [Member] \| Level 2 [Member]
Trading securities owned	123,758	66,424
Available-for-sale securities	1,113	1,103
Trading securities sold, but not yet purchased	15	954
U.S. Government Securities [Member]
Trading securities owned	3,573	32,845
Investments	7,069
Trading securities sold, but not yet purchased	162,661	109,776
U.S. Government Securities [Member] \| Level 1 [Member]
Trading securities owned	3,573	32,845
Investments	7,069
Trading securities sold, but not yet purchased	162,661	109,776
Corporate Fixed Income Securities [Member]
Trading securities owned	396,878	244,535
Available-for-sale securities	480,182	405,985
Trading securities sold, but not yet purchased	150,698	149,460
Corporate Fixed Income Securities [Member] \| Level 1 [Member]
Trading securities owned	66,795	31,398
Available-for-sale securities	263,017	153,855
Trading securities sold, but not yet purchased	46,274	74,719
Corporate Fixed Income Securities [Member] \| Level 2 [Member]
Trading securities owned	329,500	209,395
Available-for-sale securities	217,165	240,130
Trading securities sold, but not yet purchased	104,424	74,741
Corporate Fixed Income Securities [Member] \| Level 3 [Member]
Trading securities owned	583	3,742
Available-for-sale securities		12,000
Corporate Equity Securities [Member]
Trading securities owned	35,472	19,859
Investments	32,162	23,921
Trading securities sold, but not yet purchased	6,281	6,060
Corporate Equity Securities [Member] \| Level 1 [Member]
Trading securities owned	33,650	19,506
Investments	32,162	23,921
Trading securities sold, but not yet purchased	5,936	6,019
Corporate Equity Securities [Member] \| Level 2 [Member]
Trading securities owned	1,822	353
Trading securities sold, but not yet purchased	345	41
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	203,927	111,288
Available-for-sale securities	157,420	86,932
Trading securities sold, but not yet purchased	87	583
State And Municipal Securities [Member] \| Level 2 [Member]
Trading securities owned	203,927	111,288
Available-for-sale securities	66,933	20,036
Trading securities sold, but not yet purchased	87	583
State And Municipal Securities [Member] \| Level 3 [Member]
Available-for-sale securities	90,487	66,896
Agency [Member]
Available-for-sale securities	684,848	404,662
Agency [Member] \| Level 2 [Member]
Available-for-sale securities	684,848	404,662
Commercial [Member]
Available-for-sale securities	260,974	271,510
Commercial [Member] \| Level 2 [Member]
Available-for-sale securities	260,974	271,510
Non-Agency [Member]
Available-for-sale securities	13,878	17,460
Non-Agency [Member] \| Level 2 [Member]
Available-for-sale securities	13,878	17,460
Asset-Backed Securities [Member]
Available-for-sale securities	26,753	26,489
Asset-Backed Securities [Member] \| Level 2 [Member]
Available-for-sale securities	26,753	26,489
Mutual Funds [Member]
Investments	18,021	33,958
Mutual Funds [Member] \| Level 1 [Member]
Investments	18,021	33,958
Securities Sold, But Not yet Purchased [Member]
Trading securities sold, but not yet purchased	22,966	19,223
Securities Sold, But Not yet Purchased [Member] \| Level 1 [Member]
Trading securities sold, but not yet purchased	22,966	19,223
Auction Rate Equity Securities [Member]
Investments	64,397	103,176
Auction Rate Equity Securities [Member] \| Level 3 [Member]
Investments	64,397	103,176
Auction Rate Municipal Securities [Member]
Investments	14,067	11,729
Auction Rate Municipal Securities [Member] \| Level 3 [Member]
Investments	14,067	11,729
Other Invesment [Member]
Investments	43,748	38,424
Other Invesment [Member] \| Level 1 [Member]
Investments	1,620	1,055
Other Invesment [Member] \| Level 2 [Member]
Investments	4,831	336
Other Invesment [Member] \| Level 3 [Member]
Investments	$ 37,297	$ 37,033

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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- Definition

Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

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- Definition

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- Definition

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- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Trading Securities which consist of all investments in certain debt and equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

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