Related Party (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 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			550
Accounts receivable			2,255
Accounts payable			86
Summary of transactions with the Stockholder Broker-Dealer Client or its affiliates
Commissions	1,480	5,080
Technology products and services	35	115
Information and user access fees	24	140
Investment Income	20	66
Other Income	11	51
General and administrative	$ 7	$ 60