Receivables and Payables (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 711,575,000		$ 711,575,000		$ 297,916,000
Clearing organizations	5,627,000		5,627,000		12,391,000
Securities borrowed	928,899,000		928,899,000		798,228,000
Allowance for doubtful accounts	(1,217,000)		(1,217,000)		(1,416,000)
Total	1,644,884,000		1,644,884,000		1,107,119,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	462,042,000		462,042,000		513,529,000
Clearing organizations	93,806,000		93,806,000		728,000
Securities loaned	966,595,000		966,595,000		823,202,000
Total	1,522,443,000		1,522,443,000		1,337,459,000
Receivables from customers
Customers	943,154,000		943,154,000		548,287,000
Allowance for doubtful accounts	(904,000)		(904,000)		(1,462,000)
Total	942,250,000		942,250,000		546,825,000
Payables to customers
Customers	936,381,000		936,381,000		226,892,000
Total	936,381,000		936,381,000		226,892,000
Securities Borrowed and Loaned
Fair value of securities borrowed	905,900,000		905,900,000
Interest earned	2,882,000	3,799,000	11,662,000	17,335,000
Interest incurred	(1,938,000)	(2,680,000)	(8,242,000)	(13,332,000)
Interest earned (incurred), net	944,000	1,119,000	3,420,000	4,003,000
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	928,899,000		928,899,000		798,228,000
Net Amounts Presented in the Condensed Consolidated Statement of Financial Condition	928,899,000		928,899,000		798,228,000
Collateral Received or Pledged (including Cash)	899,661,000		899,661,000		789,317,000
Net Amount	29,238,000		29,238,000		8,911,000
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(966,595,000)		(966,595,000)		(823,202,000)
Net Amounts Presented in the Condensed Consolidated Statement of Financial Condition	(966,595,000)		(966,595,000)		(823,202,000)
Collateral Received or Pledged (including Cash)	(966,595,000)		(966,595,000)		(822,382,000)
Net Amount					$ (820,000)