Related Parties (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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	7,045	5,541
Technology products and services	35	167	16
Information and user access fees	24	176	132
Investment Income	20	90	113
Other Income	11	66	63
General and administrative	$ 7	$ 73	$ 27