Fair Value Of Financial Instruments (Schedule Of Valuation Of Financial Instruments By Pricing Observability Levels) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Cash and cash equivalents	$ 62,045	$ 14,156
Trading securities owned	750,947	474,951
Available-for-sale securities	1,329,359	1,214,141
Investments	180,080	211,208
Total Assets	2,322,431	1,914,456
Trading securities sold, but not yet purchased	411,680	266,833
Derivative contracts, Liabilities	23,207	24,877
Total Liabilities	455,744	310,933
Level 1 [Member]
Cash and cash equivalents	62,045	14,156
Trading securities owned	159,872	83,749
Available-for-sale securities	288,066	153,855
Investments	48,971	58,934
Total Assets	558,954	310,694
Trading securities sold, but not yet purchased	289,055	190,514
Total Liabilities	309,912	209,737
Level 2 [Member]
Trading securities owned	578,267	387,460
Available-for-sale securities	928,563	981,390
Investments	478	336
Total Assets	1,507,308	1,369,186
Trading securities sold, but not yet purchased	122,625	76,319
Derivative contracts, Liabilities	23,207	24,877
Total Liabilities	145,832	101,196
Level 3 [Member]
Trading securities owned	12,808	3,742
Available-for-sale securities	112,730	78,896
Investments	130,631	151,938
Total Assets	256,169	234,576
U.S. Government Agency Securities [Member]
Trading securities owned	80,673	66,424
Available-for-sale securities	1,115	1,103
Trading securities sold, but not yet purchased	6,658	954
U.S. Government Agency Securities [Member] \| Level 2 [Member]
Trading securities owned	80,673	66,424
Available-for-sale securities	1,115	1,103
Trading securities sold, but not yet purchased	6,658	954
U.S. Government Securities [Member]
Trading securities owned	31,879	32,845
Trading securities sold, but not yet purchased	208,786	109,776
U.S. Government Securities [Member] \| Level 1 [Member]
Trading securities owned	31,879	32,845
Trading securities sold, but not yet purchased	208,786	109,776
Corporate Fixed Income Securities [Member]
Trading securities owned	432,086	244,535
Available-for-sale securities	470,981	405,985
Trading securities sold, but not yet purchased	177,976	149,460
Corporate Fixed Income Securities [Member] \| Level 1 [Member]
Trading securities owned	85,059	31,398
Available-for-sale securities	288,066	153,855
Trading securities sold, but not yet purchased	62,330	74,719
Corporate Fixed Income Securities [Member] \| Level 2 [Member]
Trading securities owned	334,219	209,395
Available-for-sale securities	170,915	240,130
Trading securities sold, but not yet purchased	115,646	74,741
Corporate Fixed Income Securities [Member] \| Level 3 [Member]
Trading securities owned	12,808	3,742
Available-for-sale securities	12,000	12,000
Corporate Equity Securities [Member]
Trading securities owned	43,997	19,859
Investments	25,473	23,921
Trading securities sold, but not yet purchased	18,090	6,060
Corporate Equity Securities [Member] \| Level 1 [Member]
Trading securities owned	42,934	19,506
Investments	25,358	23,921
Trading securities sold, but not yet purchased	17,939	6,019
Corporate Equity Securities [Member] \| Level 2 [Member]
Trading securities owned	1,063	353
Investments	115
Trading securities sold, but not yet purchased	151	41
State And Municipal Securities [Member]
Trading securities owned	162,312	111,288
Available-for-sale securities	119,603	86,932
Trading securities sold, but not yet purchased	170	583
State And Municipal Securities [Member] \| Level 2 [Member]
Trading securities owned	162,312	111,288
Available-for-sale securities	18,873	20,036
Trading securities sold, but not yet purchased	170	583
State And Municipal Securities [Member] \| Level 3 [Member]
Available-for-sale securities	100,730	66,896
Agency [Member]
Available-for-sale securities	411,277	404,662
Agency [Member] \| Level 2 [Member]
Available-for-sale securities	411,277	404,662
Commercial [Member]
Available-for-sale securities	283,825	271,510
Commercial [Member] \| Level 2 [Member]
Available-for-sale securities	283,825	271,510
Non-Agency [Member]
Available-for-sale securities	15,408	17,460
Non-Agency [Member] \| Level 2 [Member]
Available-for-sale securities	15,408	17,460
Asset-Backed Securities [Member]
Available-for-sale securities	27,150	26,489
Asset-Backed Securities [Member] \| Level 2 [Member]
Available-for-sale securities	27,150	26,489
Mutual Funds [Member]
Investments	15,836	33,958
Mutual Funds [Member] \| Level 1 [Member]
Investments	15,836	33,958
Securities Sold, But Not yet Purchased [Member]
Trading securities sold, but not yet purchased	20,857	19,223
Securities Sold, But Not yet Purchased [Member] \| Level 1 [Member]
Trading securities sold, but not yet purchased	20,857	19,223
Auction Rate Equity Securities [Member]
Investments	80,037	103,176
Auction Rate Equity Securities [Member] \| Level 3 [Member]
Investments	80,037	103,176
Auction Rate Municipal Securities [Member]
Investments	11,503	11,729
Auction Rate Municipal Securities [Member] \| Level 3 [Member]
Investments	11,503	11,729
Other [Member]
Investments	40,163	38,424
Other [Member] \| Level 1 [Member]
Investments	709	1,055
Other [Member] \| Level 2 [Member]
Investments	363	336
Other [Member] \| Level 3 [Member]
Investments	$ 39,091	$ 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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