Receivables and Payables (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 578,565,000		$ 297,916,000
Clearing organizations	155,138,000		12,391,000
Securities borrowed	1,379,683,000		798,228,000
Allowance for doubtful accounts	(1,344,000)		(1,416,000)
Total	2,112,042,000		1,107,119,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	934,515,000		513,529,000
Clearing organizations	49,061,000		728,000
Securities loaned	1,428,514,000		823,202,000
Total	2,412,090,000		1,337,459,000
Receivables from customers
Customers	1,363,423,000		548,287,000
Allowance for doubtful accounts	(1,382,000)		(1,462,000)
Total	1,362,041,000		546,825,000
Payables to customers
Customers	935,072,000		226,892,000
Total	935,072,000		226,892,000
Securities Borrowed and Loaned
Fair value of securities borrowed	1,400,000,000
Interest earned	2,926,000	5,902,000
Interest incurred	(2,053,000)	(4,505,000)
Interest earned incurred, net	873,000	1,397,000
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	1,379,683,000		798,228,000
Net Amounts Presented in the Statement of Financial Position	1,379,683,000		798,228,000
Gross Amounts Not Offset in the Statement of Financial Position - Financial Instruments	1,355,824,000		789,317,000
Net Amount	23,859,000		8,911,000
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(1,428,514,000)		(823,202,000)
Net Amounts Presented in the Statement of Financial Position	(1,428,514,000)		(823,202,000)
Gross Amounts Not Offset in the Statement of Financial Position - Financial Instruments	(1,428,488,000)		(822,382,000)
Net Amount	$ (26,000)		$ (820,000)