Fair Value of Financial Instruments (Schedule of Fair Value of Financial Instruments) (Details) (USD $) In Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2011		Dec. 31, 2010
Assets
Cash and cash equivalents	$ 7,992		$ 15,675
Securities purchased under agreements to resell	97,685		123,617
Trading securities owned	603,964		444,170
Available-for-sale Securities	1,074,114		1,012,714
Investments	167,495		178,936
Securities sold under agreements to repurchase	42,464		109,595
Trading securities sold, but not yet purchased	291,465		200,140
Derivative contracts	11,857	[1]
Carrying Value [Member]
Assets
Cash and cash equivalents	150,964		253,529
Restricted cash	6,876		6,868
Cash segregated for regulatory purposes	25		6,023
Securities purchased under agreements to resell	97,685		123,617
Trading securities owned	603,964		444,170
Available-for-sale Securities	1,074,114		1,012,714
Held-to-maturity securities	103,736		52,640
Loans held for sale	55,110		86,344
Bank loans	478,006		389,742
Investments	167,495		178,936
Securities sold under agreements to repurchase	42,464		109,595
Non-interest-bearing deposits	11,017		8,197
Trading securities sold, but not yet purchased	291,465		200,140
Interest-bearing deposits	1,630,062		1,615,371
Derivative contracts	11,857	[1]	9,259	[1]
Securities sold, but not yet purchased	21,641	[2]	19,935	[2]
Liabilities subordinated to the claims of general creditors	6,957		8,241
Estimated Fair Value [Member]
Assets
Cash and cash equivalents	150,964		253,529
Restricted cash	6,876		6,868
Cash segregated for regulatory purposes	25		6,023
Securities purchased under agreements to resell	97,685		123,617
Trading securities owned	603,964		444,170
Available-for-sale Securities	1,074,114		1,012,714
Held-to-maturity securities	102,632		52,984
Loans held for sale	55,110		86,344
Bank loans	480,554		376,176
Investments	167,495		178,936
Securities sold under agreements to repurchase	42,464		109,595
Non-interest-bearing deposits	10,904		7,980
Trading securities sold, but not yet purchased	291,465		200,140
Interest-bearing deposits	1,588,314		1,565,199
Derivative contracts	11,857	[1]	9,259	[1]
Securities sold, but not yet purchased	21,641	[2]	19,935	[2]
Liabilities subordinated to the claims of general creditors	$ 6,563		$ 7,739

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

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