Receivables and Payables (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 297,916,000	$ 205,975,000
Clearing organizations	12,391,000	2,365,000
Securities borrowed	798,228,000	663,293,000
Allowance for doubtful accounts	(1,416,000)	(318,000)
Total	1,107,119,000	871,315,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	513,529,000	370,146,000
Clearing organizations	728,000	14,945,000
Securities loaned	823,202,000	694,682,000
Total	1,337,459,000	1,079,773,000
Receivables from customers
Customers	548,287,000	473,852,000
Allowance for doubtful accounts	(1,462,000)	(1,343,000)
Total	546,825,000	472,509,000
Payables to customers
Customers	226,892,000	207,738,000
Total	226,892,000	207,738,000
Allowance for doubtful accounts
Increase in allowance for doubtful debts	1,401,000	203,000	178,000
Total write-offs against the allowance	200,000	100,000	100,000
Securities Borrowed and Loaned
Fair value of securities borrowed	790,800,000
Interest earned	19,242,000	19,130,000
Interest incurred	(14,589,000)	(14,646,000)
Interest earned incurred, net	$ 4,653,000	$ 4,484,000