Fair Value of Financial Instruments (Schedule of Fair Value of Financial Instruments) (Details) (USD $) In Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Assets
Cash and cash equivalents	$ 16,152		$ 15,675
Securities purchased under agreements to resell	121,004		123,617
Trading securities owned	526,444		444,170
Available-for-sale Securities			1,012,714
Investments	176,550		178,936
Securities sold under agreements to repurchase	52,805		109,595
Trading securities sold, but not yet purchased	272,190		200,140
Derivative contracts	25,672	[1]
Carrying Value [Member]
Assets
Cash and cash equivalents	214,619		253,529
Restricted cash	6,880		6,868
Cash segregated for regulatory purposes	26		6,023
Securities purchased under agreements to resell	121,004		123,617
Trading securities owned	526,444		444,170
Available-for-sale Securities	1,312,784		1,012,714
Held-to-maturity securities	159,132		52,640
Loans held for sale	114,452		86,344
Bank loans	568,293		389,742
Investments	176,550		178,936
Securities sold under agreements to repurchase	52,805		109,595
Non-interest-bearing deposits	21,405		8,197
Trading securities sold, but not yet purchased	272,190		200,140
Interest-bearing deposits	2,099,358		1,615,371
Derivative contracts	25,672	[2]	9,259	[2]
Securities sold, but not yet purchased	16,815	[1]	19,935	[1]
Liabilities subordinated to the claims of general creditors	6,957		8,241
Estimated Fair Value [Member]
Assets
Cash and cash equivalents	214,619		253,529
Restricted cash	6,880		6,868
Cash segregated for regulatory purposes	26		6,023
Securities purchased under agreements to resell	121,004		123,617
Trading securities owned	526,444		444,170
Available-for-sale Securities	1,312,784		1,012,714
Held-to-maturity securities	157,085		52,984
Loans held for sale	114,452		86,344
Bank loans	571,716		376,176
Investments	176,550		178,936
Securities sold under agreements to repurchase	52,805		109,595
Non-interest-bearing deposits	21,398		7,980
Trading securities sold, but not yet purchased	272,190		200,140
Interest-bearing deposits	2,072,029		1,565,199
Derivative contracts	25,672	[2]	9,259	[2]
Securities sold, but not yet purchased	16,815	[1]	19,935	[1]
Liabilities subordinated to the claims of general creditors	$ 6,617		$ 7,739

[1] Included in other liabilities in the consolidated statements of financial condition.

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