Receivables and Payables (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 336,388,000		$ 336,388,000		$ 205,975,000
Clearing organizations	7,085,000		7,085,000		2,365,000
Securities borrowed	601,621,000		601,621,000		663,293,000
Allowance for doubtful accounts	(969,000)		(969,000)		(318,000)
Total	944,125,000		944,125,000		871,315,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	386,666,000		386,666,000		370,146,000
Clearing organizations	3,353,000		3,353,000		14,945,000
Securities loaned	614,887,000		614,887,000		694,682,000
Total	1,004,906,000		1,004,906,000		1,079,773,000
Receivables from customers
Customers	590,817,000		590,817,000		473,852,000
Allowance for doubtful accounts	(1,483,000)		(1,483,000)		(1,343,000)
Total	589,334,000		589,334,000		472,509,000
Payables to customers
Customers	440,396,000		440,396,000		207,738,000
Total	440,396,000		440,396,000		207,738,000
Securities Borrowed and Loaned
Fair value of securities borrowed	591,000,000		591,000,000
Interest earned	7,634,000	6,328,000	13,537,000	9,680,000
Interest incurred	(6,147,000)	(5,003,000)	(10,652,000)	(7,420,000)
Interest earned incurred, net	$ 1,487,000	$ 1,325,000	$ 2,885,000	$ 2,260,000