Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Revenues	$ 511,352	$ 435,565	$ 393,422
Expenses
Employee compensation and benefits	131,079	109,117	102,313
Depreciation and amortization	26,857	23,080	19,274
Technology and communications	26,792	23,866	20,048
Professional and consulting fees	25,534	21,521	19,367
Occupancy	11,639	14,176	6,125
Marketing and advertising	11,559	12,114	9,762
Clearing costs	11,314	7,754	5,797
General and administrative	15,696	11,353	11,121
Total expenses	260,470	222,981	193,807
Operating income	250,882	212,584	199,615
Other income (expense)
Investment income	8,063	6,112	3,619
Other, net	(1,521)	(610)	(1,466)
Total other income	6,542	5,502	2,153
Income before income taxes	257,424	218,086	201,768
Provision for income taxes	52,522	45,234	53,679
Net income	$ 204,902	$ 172,852	$ 148,089
Net income per common share
Basic	$ 5.53	$ 4.68	$ 4.02
Diluted	5.40	4.57	3.89
Cash dividends declared per common share	$ 2.04	$ 1.68	$ 1.32
Weighted average shares outstanding
Basic	37,083	36,958	36,864
Diluted	37,956	37,855	38,038
Commissions [Member]
Revenues
Revenues	$ 463,856	$ 390,834	$ 355,282
Information Services [Member]
Revenues
Revenues	30,730	28,227	25,806
Post-trade Services [Member]
Revenues
Revenues	15,763	15,346	11,090
Other [Member]
Revenues
Revenues	$ 1,003	$ 1,158	$ 1,244