Fair Value Of Financial Instruments (Schedule Of Valuation Of Financial Instruments By Pricing Observability Levels) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Cash and cash equivalents	$ 26,675	$ 14,156
Trading securities owned	746,213	474,951
Available-for-sale securities	1,476,681	1,214,141
Investments	214,761	211,208
Total Assets	2,464,330	1,914,456
Trading securities sold, but not yet purchased	369,886	266,833
Derivative contracts, Liabilities	22,455	24,877
Total Liabilities	414,535	310,933
Level 1 [Member]
Cash and cash equivalents	26,675	14,156
Trading securities owned	113,546	83,749
Available-for-sale securities	331,244	153,855
Investments	51,782	58,934
Total Assets	523,247	310,694
Trading securities sold, but not yet purchased	232,618	190,514
Total Liabilities	254,812	209,737
Level 2 [Member]
Trading securities owned	629,196	387,460
Available-for-sale securities	1,049,812	981,390
Investments	47,597	336
Total Assets	1,726,605	1,369,186
Trading securities sold, but not yet purchased	137,268	76,319
Derivative contracts, Liabilities	22,455	24,877
Total Liabilities	159,723	101,196
Level 3 [Member]
Trading securities owned	3,471	3,742
Available-for-sale securities	95,625	78,896
Investments	115,382	151,938
Total Assets	214,478	234,576
U.S. Government Agency Securities [Member]
Trading securities owned	144,286	66,424
Available-for-sale securities	1,115	1,103
Trading securities sold, but not yet purchased	8,464	954
U.S. Government Agency Securities [Member] \| Level 2 [Member]
Trading securities owned	144,286	66,424
Available-for-sale securities	1,115	1,103
Trading securities sold, but not yet purchased	8,464	954
U.S. Government Securities [Member]
Trading securities owned	8,059	32,845
Investments	7,068
Trading securities sold, but not yet purchased	148,876	109,776
U.S. Government Securities [Member] \| Level 1 [Member]
Trading securities owned	8,059	32,845
Investments	7,068
Trading securities sold, but not yet purchased	148,876	109,776
Corporate Fixed Income Securities [Member]
Trading securities owned	391,981	244,535
Available-for-sale securities	500,982	405,985
Trading securities sold, but not yet purchased	173,157	149,460
Corporate Fixed Income Securities [Member] \| Level 1 [Member]
Trading securities owned	68,005	31,398
Available-for-sale securities	331,244	153,855
Trading securities sold, but not yet purchased	45,814	74,719
Corporate Fixed Income Securities [Member] \| Level 2 [Member]
Trading securities owned	320,505	209,395
Available-for-sale securities	158,209	240,130
Trading securities sold, but not yet purchased	127,343	74,741
Corporate Fixed Income Securities [Member] \| Level 3 [Member]
Trading securities owned	3,471	3,742
Available-for-sale securities	11,529	12,000
Corporate Equity Securities [Member]
Trading securities owned	39,829	19,859
Investments	26,111	23,921
Trading securities sold, but not yet purchased	39,284	6,060
Corporate Equity Securities [Member] \| Level 1 [Member]
Trading securities owned	37,482	19,506
Investments	25,413	23,921
Trading securities sold, but not yet purchased	37,928	6,019
Corporate Equity Securities [Member] \| Level 2 [Member]
Trading securities owned	2,347	353
Investments	698
Trading securities sold, but not yet purchased	1,356	41
Corporate Preferred Securities [Member]
Investments	46,558
Corporate Preferred Securities [Member] \| Level 2 [Member]
Investments	46,558
State And Municipal Securities [Member]
Trading securities owned	162,058	111,288
Available-for-sale securities	142,586	86,932
Trading securities sold, but not yet purchased	105	583
State And Municipal Securities [Member] \| Level 2 [Member]
Trading securities owned	162,058	111,288
Available-for-sale securities	58,490	20,036
Trading securities sold, but not yet purchased	105	583
State And Municipal Securities [Member] \| Level 3 [Member]
Available-for-sale securities	84,096	66,896
Agency [Member]
Available-for-sale securities	520,332	404,662
Agency [Member] \| Level 2 [Member]
Available-for-sale securities	520,332	404,662
Commercial [Member]
Available-for-sale securities	269,808	271,510
Commercial [Member] \| Level 2 [Member]
Available-for-sale securities	269,808	271,510
Non-Agency [Member]
Available-for-sale securities	14,765	17,460
Non-Agency [Member] \| Level 2 [Member]
Available-for-sale securities	14,765	17,460
Asset-Backed Securities [Member]
Available-for-sale securities	27,093	26,489
Asset-Backed Securities [Member] \| Level 2 [Member]
Available-for-sale securities	27,093	26,489
Mutual Funds [Member]
Investments	17,750	33,958
Mutual Funds [Member] \| Level 1 [Member]
Investments	17,750	33,958
Securities Sold, But Not yet Purchased [Member]
Trading securities sold, but not yet purchased	22,194	19,223
Securities Sold, But Not yet Purchased [Member] \| Level 1 [Member]
Trading securities sold, but not yet purchased	22,194	19,223
Auction Rate Equity Securities [Member]
Investments	67,887	103,176
Auction Rate Equity Securities [Member] \| Level 3 [Member]
Investments	67,887	103,176
Auction Rate Municipal Securities [Member]
Investments	10,778	11,729
Auction Rate Municipal Securities [Member] \| Level 3 [Member]
Investments	10,778	11,729
Other [Member]
Investments	38,609	38,424
Other [Member] \| Level 1 [Member]
Investments	1,551	1,055
Other [Member] \| Level 2 [Member]
Investments	341	336
Other [Member] \| Level 3 [Member]
Investments	$ 36,717	$ 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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