Related Party (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Summary of balances with Stockholder Broker-Dealer Client or affiliates
Cash and cash equivalents			$ 156,982
Securities and cash provided as collateral Related Parties			550
Accounts receivable			2,255
Accounts payable			86
Summary of transactions with the Stockholder Broker-Dealer Client or its affiliates
Commissions	1,480	1,719
Technology products and services	35
Information and user access fees	24	47
Investment Income	20	25
Other Income	11	17
General and administrative	$ 7	$ 20