Fair Value Measurements (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Assets
Securities owned, at fair value:	$ 5,890	$ 25,789
Liabilities
Securities sold, not yet purchased, at fair value:	1,395	19,362
Recurring basis | Total
Assets
Total Assets	145,373	231,438
Liabilities
Total Liabilities	1,407	19,371
Recurring basis | Total | Tax free money market mutual funds
Assets
Cash and cash equivalents:	4,590	5,061
Recurring basis | Total | U.S. government money market mutual funds
Assets
Cash and cash equivalents:	128,845	192,617
Recurring basis | Total | Money market mutual funds
Assets
Cash and cash equivalents:	6,048	7,971
Recurring basis | Total | Corporate stocks - trading securities
Assets
Securities owned, at fair value:	1,348	19,051
Recurring basis | Total | Corporate stocks - available-for-sale
Assets
Securities owned, at fair value:		1,662
Recurring basis | Total | Mutual funds
Assets
Securities owned, at fair value:	4,542	5,076
Recurring basis | Total | Currency forward contracts
Liabilities
Accounts payable and accrued expenses:	12	9
Recurring basis | Total | Common Stock
Liabilities
Securities sold, not yet purchased, at fair value:	1,395	19,362
Recurring basis | Level 1
Assets
Total Assets	145,373	231,438
Liabilities
Total Liabilities	1,395	19,362
Recurring basis | Level 1 | Tax free money market mutual funds
Assets
Cash and cash equivalents:	4,590	5,061
Recurring basis | Level 1 | U.S. government money market mutual funds
Assets
Cash and cash equivalents:	128,845	192,617
Recurring basis | Level 1 | Money market mutual funds
Assets
Cash and cash equivalents:	6,048	7,971
Recurring basis | Level 1 | Corporate stocks - trading securities
Assets
Securities owned, at fair value:	1,348	19,051
Recurring basis | Level 1 | Corporate stocks - available-for-sale
Assets
Securities owned, at fair value:		1,662
Recurring basis | Level 1 | Mutual funds
Assets
Securities owned, at fair value:	4,542	5,076
Recurring basis | Level 1 | Common Stock
Liabilities
Securities sold, not yet purchased, at fair value:	1,395	19,362
Recurring basis | Level 2
Liabilities
Total Liabilities	1,200	9
Recurring basis | Level 2 | Currency forward contracts
Liabilities
Accounts payable and accrued expenses:	$ 12	$ 9