Fair Value Of Financial Instruments (Schedule Of Valuation Of Financial Instruments By Pricing Observability Levels) (Details) (USD $)	Mar. 31, 2012		Dec. 31, 2011
Cash equivalents	$ 48,751,000		$ 14,156,000
Trading securities owned	575,724,000		474,951,000
Available-for-sale securities	1,347,535,000		1,214,141,000
Investments	216,106,000		211,208,000
Total Assets	2,188,116,000		1,914,456,000
Trading securities sold, but not yet purchased	267,807,000		266,833,000
Derivative contracts, Liabilities	21,040,000		24,877,000
Total Liabilities	310,670,000		310,933,000
Level 1 [Member]
Cash equivalents	48,751,000		14,156,000
Trading securities owned	111,388,000		83,749,000
Available-for-sale securities	310,850,000		153,855,000
Investments	72,108,000		58,934,000
Total Assets	543,097,000		310,694,000
Trading securities sold, but not yet purchased	162,984,000		190,514,000
Total Liabilities	184,807,000		209,737,000
Level 2 [Member]
Trading securities owned	457,639,000		387,460,000
Available-for-sale securities	959,826,000		981,390,000
Investments	535,000		336,000
Total Assets	1,418,000,000		1,369,186,000
Trading securities sold, but not yet purchased	104,823,000		76,319,000
Derivative contracts, Liabilities	21,040,000		24,877,000
Total Liabilities	125,863,000		101,196,000
Level 3 [Member]
Trading securities owned	6,697,000		3,742,000
Available-for-sale securities	76,859,000		78,896,000
Investments	143,463,000		151,938,000
Total Assets	227,019,000		234,576,000
U.S. Government Agency Securities [Member]
Trading securities owned	91,271,000		66,424,000
Available-for-sale securities			1,103,000
Trading securities sold, but not yet purchased	6,161,000		954,000
U.S. Government Agency Securities [Member] | Level 2 [Member]
Trading securities owned	91,271,000		66,424,000
Available-for-sale securities			1,103,000
Trading securities sold, but not yet purchased	6,161,000		954,000
U.S. Government Securities [Member]
Trading securities owned	15,030,000		32,845,000
Available-for-sale securities	1,116,000
Investments	7,000,000
Trading securities sold, but not yet purchased	98,283,000		109,776,000
U.S. Government Securities [Member] | Level 1 [Member]
Trading securities owned	15,030,000		32,845,000
Investments	7,000,000
Trading securities sold, but not yet purchased	98,283,000		109,776,000
U.S. Government Securities [Member] | Level 2 [Member]
Available-for-sale securities	1,116,000
State And Municipal Securities [Member]
Available-for-sale securities	83,486,000		86,932,000
Trading securities sold, but not yet purchased	389,000		583,000
State And Municipal Securities [Member] | Level 2 [Member]
Available-for-sale securities	18,607,000		20,036,000
Trading securities sold, but not yet purchased	389,000		583,000
State And Municipal Securities [Member] | Level 3 [Member]
Available-for-sale securities	64,859,000		66,896,000
Corporate Fixed Income Securities [Member]
Available-for-sale securities	468,097,000		405,985,000
Trading securities sold, but not yet purchased	145,002,000		149,460,000
Corporate Fixed Income Securities [Member] | Level 1 [Member]
Available-for-sale securities	310,850,000		153,855,000
Trading securities sold, but not yet purchased	46,729,000		74,719,000
Corporate Fixed Income Securities [Member] | Level 2 [Member]
Available-for-sale securities	145,247,000		240,130,000
Trading securities sold, but not yet purchased	98,273,000		74,741,000
Corporate Fixed Income Securities [Member] | Level 3 [Member]
Available-for-sale securities	12,000,000		12,000,000
Agency [Member]
Available-for-sale securities	465,882,000		404,662,000
Agency [Member] | Level 2 [Member]
Available-for-sale securities	465,882,000		404,662,000
Commercial [Member]
Available-for-sale securities	286,868,000		271,510,000
Commercial [Member] | Level 2 [Member]
Available-for-sale securities	286,868,000		271,510,000
Non-Agency [Member]
Available-for-sale securities	16,689,000		17,460,000
Non-Agency [Member] | Level 2 [Member]
Available-for-sale securities	16,689,000		17,460,000
Asset-Backed Securities [Member]
Available-for-sale securities	25,417,000		26,489,000
Asset-Backed Securities [Member] | Level 2 [Member]
Available-for-sale securities	25,417,000		26,489,000
Corporate Equity Securities [Member]
Investments	27,504,000		23,921,000
Corporate Equity Securities [Member] | Level 1 [Member]
Investments	27,311,000		23,921,000
Corporate Equity Securities [Member] | Level 2 [Member]
Investments	193,000
Mutual Funds [Member]
Investments	36,855,000		33,958,000
Mutual Funds [Member] | Level 1 [Member]
Investments	36,855,000		33,958,000
Other [Member]
Investments			38,424,000
Other [Member] | Level 1 [Member]
Investments			1,055,000
Other [Member] | Level 2 [Member]
Investments			336,000
Other [Member] | Level 3 [Member]
Investments			37,033,000
Securities Sold, Not yet Purchased [Member]
Trading securities sold, but not yet purchased	21,823,000	[1]	19,223,000	[1]
Securities Sold, Not yet Purchased [Member] | Level 1 [Member]
Trading securities sold, but not yet purchased	21,823,000	[1]	19,223,000	[1]
Auction Rate Equity Securities [Member]
Investments	4.7		103,176,000
Trading securities sold, but not yet purchased	17,972,000		6,060,000
Auction Rate Equity Securities [Member] | Level 1 [Member]
Trading securities sold, but not yet purchased	17,972,000		6,019,000
Auction Rate Equity Securities [Member] | Level 2 [Member]
Trading securities sold, but not yet purchased			41,000
Auction Rate Equity Securities [Member] | Level 3 [Member]
Investments			103,176,000
Auction Rate Municipal Securities [Member]
Investments	4.2		11,729,000
Auction Rate Municipal Securities [Member] | Level 3 [Member]
Investments			11,729,000
Auction Rate Securities [Member] | Other [Member]
Investments	39,266,000
Auction Rate Securities [Member] | Other [Member] | Level 1 [Member]
Investments	942,000
Auction Rate Securities [Member] | Other [Member] | Level 2 [Member]
Investments	342,000
Auction Rate Securities [Member] | Other [Member] | Level 3 [Member]
Investments	37,982,000
Auction Rate Securities [Member] | Auction Rate Equity Securities [Member]
Investments	92,077,000
Auction Rate Securities [Member] | Auction Rate Equity Securities [Member] | Level 3 [Member]
Investments	92,077,000
Auction Rate Securities [Member] | Auction Rate Municipal Securities [Member]
Investments	13,404,000
Auction Rate Securities [Member] | Auction Rate Municipal Securities [Member] | Level 3 [Member]
Investments	13,404,000
Corporate Securities [Member] | State And Municipal Securities [Member]
Trading securities owned	197,369,000		111,288,000
Corporate Securities [Member] | State And Municipal Securities [Member] | Level 2 [Member]
Trading securities owned	197,369,000		111,288,000
Corporate Securities [Member] | Corporate Fixed Income Securities [Member]
Trading securities owned	240,244,000		244,535,000
Corporate Securities [Member] | Corporate Fixed Income Securities [Member] | Level 1 [Member]
Trading securities owned	67,774,000		31,398,000
Corporate Securities [Member] | Corporate Fixed Income Securities [Member] | Level 2 [Member]
Trading securities owned	165,773,000		209,395,000
Corporate Securities [Member] | Corporate Fixed Income Securities [Member] | Level 3 [Member]
Trading securities owned	6,697,000		3,742,000
Corporate Securities [Member] | Corporate Equity Securities [Member]
Trading securities owned	31,810,000
Corporate Securities [Member] | Corporate Equity Securities [Member] | Level 1 [Member]
Trading securities owned	28,584,000
Corporate Securities [Member] | Corporate Equity Securities [Member] | Level 2 [Member]
Trading securities owned	3,226,000
Corporate Securities [Member] | Auction Rate Equity Securities [Member]
Trading securities owned			19,859,000
Corporate Securities [Member] | Auction Rate Equity Securities [Member] | Level 1 [Member]
Trading securities owned			19,506,000
Corporate Securities [Member] | Auction Rate Equity Securities [Member] | Level 2 [Member]
Trading securities owned			$ 353,000

[1] Included in accounts payable and accrued expenses in the consolidated statements of financial condition.