Related Party (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of balances with Stockholder Broker-Dealer Client or affiliates
Cash and cash equivalents	$ 156,982	$ 110,642
Securities and cash provided as collateral	550	4,049
Accounts receivable	2,255	829
Accounts payable	86	66
Summary of transactions with the Stockholder Broker-Dealer Client or its affiliates
Commissions	7,045	5,541	14,103
Technology products and services	167	16	35
Information and user access fees	176	132	243
Investment income	90	113	214
Other income	66	63	152
General and administrative	$ 73	$ 27	$ 79