Receivables and Payables (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Receivables from brokers, dealers and clearing organizations
Broker-dealers	$ 881,343,000		$ 881,343,000		$ 254,031,000
Clearing organizations	331,668,000		331,668,000		23,033,000
Securities borrowed	528,827,000		528,827,000		742,307,000
Allowance for doubtful accounts	(1,038,000)		(1,038,000)		(1,029,000)
Total	1,740,800,000		1,740,800,000		1,018,342,000
Payables to brokers, dealers and clearing organizations
Broker-dealers	1,128,417,000		1,128,417,000		284,681,000
Clearing organizations	9,041,000		9,041,000		2,798,000
Securities loaned	572,239,000		572,239,000		737,789,000
Total	1,709,697,000		1,709,697,000		1,025,268,000
Receivables from customers
Customers	1,337,833,000		1,337,833,000		592,139,000
Allowance for doubtful accounts	(1,298,000)		(1,298,000)		(1,135,000)
Net	1,336,535,000		1,336,535,000		591,004,000
Payables to customers
Customers	1,134,612,000		1,134,612,000		469,264,000
Net	1,134,612,000		1,134,612,000		469,264,000
Securities Borrowed and Loaned
Fair value of securities borrowed	513,900,000		513,900,000
Interest earned	6,455,000	5,855,000	10,858,000	8,781,000
Interest incurred	(4,337,000)	(4,251,000)	(7,403,000)	(6,304,000)
Interest earned (incurred), net	2,118,000	1,604,000	3,455,000	2,477,000
Deposits paid for securities borrowed
Gross Amounts of Recognized Assets	528,827,000		528,827,000		742,307,000
Net Amounts Presented in the Consolidated Statement of Financial Condition	528,827,000		528,827,000		742,307,000
Collateral Received or Pledged (including Cash)	528,557,000		528,557,000		742,083,000
Net Amount	270,000		270,000		224,000
Deposits received for securities loaned
Gross Amounts of Recognized Liabilities	(572,239,000)		(572,239,000)		(737,789,000)
Net Amounts Presented in the Consolidated Statement of Financial Condition	(572,239,000)		(572,239,000)		(737,789,000)
Collateral Received or Pledged (including Cash)	(560,232,000)		(560,232,000)		(722,091,000)
Net Amount	$ (12,007,000)		$ (12,007,000)		$ (15,698,000)