Receivables and Payables (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 540,099,000		$ 540,099,000		$ 205,975,000
Clearing organizations	89,933,000		89,933,000		2,365,000
Securities borrowed	661,876,000		661,876,000		663,293,000
Allowance for doubtful accounts	(1,506,000)		(1,506,000)		(318,000)
Total	1,290,402,000		1,290,402,000		871,315,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	490,550,000		490,550,000		370,146,000
Clearing organizations	11,000		11,000		14,945,000
Securities loaned	669,164,000		669,164,000		694,682,000
Total	1,159,725,000		1,159,725,000		1,079,773,000
Receivables from customers
Customers	545,389,000		545,389,000		473,852,000
Allowance for doubtful accounts	(1,453,000)		(1,453,000)		(1,343,000)
Total	543,936,000		543,936,000		472,509,000
Payables to customers
Customers	601,042,000		601,042,000		207,738,000
Total	601,042,000		601,042,000		207,738,000
Securities Borrowed and Loaned
Fair value of securities borrowed	628,100,000		628,100,000
Interest earned	3,799,000	5,114,000	17,335,000	14,794,000
Interest incurred	(2,680,000)	(4,053,000)	(13,332,000)	(11,473,000)
Interest earned incurred, net	$ 1,119,000	$ 1,061,000	$ 4,003,000	$ 3,321,000