Receivables and Payables (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 441,069,000		$ 441,069,000		$ 297,916,000
Clearing organizations	18,203,000		18,203,000		12,391,000
Securities borrowed	722,735,000		722,735,000		798,228,000
Allowance for doubtful accounts	(1,244,000)		(1,244,000)		(1,416,000)
Total	1,180,763,000		1,180,763,000		1,107,119,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	792,102,000		792,102,000		513,529,000
Clearing organizations	35,971,000		35,971,000		728,000
Securities loaned	739,886,000		739,886,000		823,202,000
Total	1,567,959,000		1,567,959,000		1,337,459,000
Receivables from customers
Customers	1,144,432,000		1,144,432,000		548,287,000
Allowance for doubtful accounts	(1,110,000)		(1,110,000)		(1,462,000)
Total	1,143,322,000		1,143,322,000		546,825,000
Payables to customers
Customers	647,953,000		647,953,000		226,892,000
Total	647,953,000		647,953,000		226,892,000
Securities Borrowed and Loaned
Fair value of securities borrowed	700,200,000		700,200,000
Interest earned	5,855,000	7,634,000	8,781,000	13,537,000
Interest incurred	(4,251,000)	(6,147,000)	(6,304,000)	(10,652,000)
Interest earned incurred, net	1,604,000	1,487,000	2,477,000	2,885,000
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	722,735,000		722,735,000		798,228,000
Net Amounts Presented in the Condensed Consolidated Statement of Financial Condition	722,735,000		722,735,000		798,228,000
Gross Amounts Not Offset in the Condensed Consolidated Statement of Financial Condition - Financial Instruments	697,824,000		697,824,000		789,317,000
Net Amount	24,911,000		24,911,000		8,911,000
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(739,886,000)		(739,886,000)		(823,202,000)
Net Amounts Presented in the Condensed Consolidated Statement of Financial Condition	(739,886,000)		(739,886,000)		(823,202,000)
Gross Amounts Not Offset in the Condensed Consolidated Statement of Financial Condition - Financial Instruments	(739,828,000)		(739,828,000)		(822,382,000)
Net Amount	$ (58,000)		$ (58,000)		$ (820,000)