Fair Value Of Financial Instruments (Schedule Of Valuation Of Financial Instruments By Pricing Observability Levels) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Cash and cash equivalents	$ 171,890	$ 72,596
Trading securities owned	923,452	763,608
Available-for-sale securities	1,729,837	1,625,168
Investments	268,082	236,434
Total Assets	3,093,261	2,697,806
Trading securities sold, but not yet purchased	588,922	319,742
Derivative contracts, Liabilities	17,459	19,934
Total Liabilities	630,931	362,642
Level 1 [Member]
Cash and cash equivalents	171,890	72,596
Trading securities owned	183,797	104,018
Available-for-sale securities	313,327	263,017
Investments	103,425	58,872
Total Assets	772,439	498,503
Trading securities sold, but not yet purchased	397,868	214,871
Total Liabilities	422,418	237,837
Level 2 [Member]
Trading securities owned	719,647	659,007
Available-for-sale securities	1,321,552	1,271,664
Investments	4,999	61,801
Total Assets	2,046,198	1,992,472
Trading securities sold, but not yet purchased	191,054	104,871
Derivative contracts, Liabilities	17,459	19,934
Total Liabilities	208,513	124,805
Level 3 [Member]
Trading securities owned	20,008	583
Available-for-sale securities	94,958	90,487
Investments	159,658	115,761
Total Assets	274,624	206,831
U.S. Government Agency Securities [Member]
Trading securities owned	204,162	123,758
Available-for-sale securities	1,217	1,113
Trading securities sold, but not yet purchased	5,256	15
U.S. Government Agency Securities [Member] \| Level 2 [Member]
Trading securities owned	204,162	123,758
Available-for-sale securities	1,217	1,113
Trading securities sold, but not yet purchased	5,256	15
U.S. Government Securities [Member]
Trading securities owned	27,703	3,573
Investments	7,069	7,069
Trading securities sold, but not yet purchased	187,253	162,661
U.S. Government Securities [Member] \| Level 1 [Member]
Trading securities owned	27,703	3,573
Investments	7,069	7,069
Trading securities sold, but not yet purchased	187,253	162,661
Corporate Fixed Income Securities [Member]
Trading securities owned	426,046	396,878
Available-for-sale securities	500,609	480,182
Trading securities sold, but not yet purchased	313,191	150,698
Corporate Fixed Income Securities [Member] \| Level 1 [Member]
Trading securities owned	49,761	66,795
Available-for-sale securities	313,327	263,017
Trading securities sold, but not yet purchased	132,239	46,274
Corporate Fixed Income Securities [Member] \| Level 2 [Member]
Trading securities owned	368,811	329,500
Available-for-sale securities	187,282	217,165
Trading securities sold, but not yet purchased	180,952	104,424
Corporate Fixed Income Securities [Member] \| Level 3 [Member]
Trading securities owned	7,474	583
Corporate Equity Securities [Member]
Trading securities owned	123,044	35,472
Investments	70,995	32,162
Trading securities sold, but not yet purchased	83,135	6,281
Corporate Equity Securities [Member] \| Level 1 [Member]
Trading securities owned	106,333	33,650
Investments	70,995	32,162
Trading securities sold, but not yet purchased	78,376	5,936
Corporate Equity Securities [Member] \| Level 2 [Member]
Trading securities owned	4,177	1,822
Trading securities sold, but not yet purchased	4,759	345
Corporate Equity Securities [Member] \| Level 3 [Member]
Trading securities owned	12,534
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	142,497	203,927
Available-for-sale securities	186,777	157,420
Trading securities sold, but not yet purchased	87	87
State And Municipal Securities [Member] \| Level 2 [Member]
Trading securities owned	142,497	203,927
Available-for-sale securities	91,819	66,933
Trading securities sold, but not yet purchased	87	87
State And Municipal Securities [Member] \| Level 3 [Member]
Available-for-sale securities	94,958	90,487
Agency [Member]
Available-for-sale securities	726,796	684,848
Agency [Member] \| Level 2 [Member]
Available-for-sale securities	726,796	684,848
Commercial [Member]
Available-for-sale securities	247,980	260,974
Commercial [Member] \| Level 2 [Member]
Available-for-sale securities	247,980	260,974
Non-Agency [Member]
Available-for-sale securities	6,294	13,878
Non-Agency [Member] \| Level 2 [Member]
Available-for-sale securities	6,294	13,878
Asset-Backed Securities [Member]
Available-for-sale securities	60,164	26,753
Asset-Backed Securities [Member] \| Level 2 [Member]
Available-for-sale securities	60,164	26,753
Mutual Funds [Member]
Investments	19,408	18,021
Mutual Funds [Member] \| Level 1 [Member]
Investments	19,408	18,021
Securities Sold, But Not yet Purchased [Member]
Trading securities sold, but not yet purchased	24,550	22,966
Securities Sold, But Not yet Purchased [Member] \| Level 1 [Member]
Trading securities sold, but not yet purchased	24,550	22,966
Auction Rate Equity Securities [Member]
Investments	63,401	64,397
Auction Rate Equity Securities [Member] \| Level 3 [Member]
Investments	63,401	64,397
Auction Rate Municipal Securities [Member]
Investments	13,999	14,067
Auction Rate Municipal Securities [Member] \| Level 3 [Member]
Investments	13,999	14,067
Other Investment [Member]
Investments	93,210	43,748
Other Investment [Member] \| Level 1 [Member]
Investments	5,953	1,620
Other Investment [Member] \| Level 2 [Member]
Investments	4,999	4,831
Other Investment [Member] \| Level 3 [Member]
Investments	$ 82,258	$ 37,297

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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