Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Revenues
Commissions	$ 67,018us-gaap_FeesAndCommissions	$ 51,989us-gaap_FeesAndCommissions
Information and post-trade services	7,679	8,079
Technology products and services	1,420us-gaap_TechnologyServicesRevenue	2,036us-gaap_TechnologyServicesRevenue
Investment income	183us-gaap_NetInvestmentIncome	146us-gaap_NetInvestmentIncome
Other	471us-gaap_OtherIncome	1,148us-gaap_OtherIncome
Total revenues	76,771us-gaap_Revenues	63,398us-gaap_Revenues
Expenses
Employee compensation and benefits	21,174us-gaap_LaborAndRelatedExpense	18,609us-gaap_LaborAndRelatedExpense
Depreciation and amortization	4,612us-gaap_DepreciationAndAmortization	4,121us-gaap_DepreciationAndAmortization
Technology and communications	4,338	4,492
Professional and consulting fees	3,282us-gaap_ProfessionalFees	3,972us-gaap_ProfessionalFees
Occupancy	992us-gaap_OccupancyNet	1,089us-gaap_OccupancyNet
Marketing and advertising	1,192us-gaap_MarketingAndAdvertisingExpense	1,209us-gaap_MarketingAndAdvertisingExpense
General and administrative	2,679us-gaap_GeneralAndAdministrativeExpense	2,198us-gaap_GeneralAndAdministrativeExpense
Total expenses	38,269us-gaap_OperatingCostsAndExpenses	35,690us-gaap_OperatingCostsAndExpenses
Income before income taxes	38,502	27,708
Provision for income taxes	13,909us-gaap_IncomeTaxExpenseBenefit	10,233us-gaap_IncomeTaxExpenseBenefit
Net income	$ 24,593us-gaap_NetIncomeLoss	$ 17,475us-gaap_NetIncomeLoss
Net income per common share
Basic	$ 0.67us-gaap_EarningsPerShareBasic	$ 0.47us-gaap_EarningsPerShareBasic
Diluted	$ 0.65us-gaap_EarningsPerShareDiluted	$ 0.46us-gaap_EarningsPerShareDiluted
Cash dividends declared per common share	$ 0.20	$ 0.16
Weighted average shares outstanding
Basic	36,706	37,121
Diluted	37,626	38,096

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