Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Revenues	$ 689,125	$ 511,352	$ 435,565
Expenses
Employee compensation and benefits	156,885	131,079	109,117
Depreciation and amortization	35,996	26,857	23,080
Technology and communications	34,092	26,792	23,866
Professional and consulting fees	32,304	25,534	21,521
Occupancy	13,425	11,639	14,176
Marketing and advertising	7,940	11,559	12,114
Clearing costs	21,058	11,314	7,754
General and administrative	12,697	15,696	11,353
Total expenses	314,397	260,470	222,981
Operating income	374,728	250,882	212,584
Other income (expense)
Investment income	2,446	8,063	6,112
Interest expense	(1,142)
Other, net	(1,673)	(1,521)	(610)
Total other income (expense)	(369)	6,542	5,502
Income before income taxes	374,359	257,424	218,086
Provision for income taxes	74,982	52,522	45,234
Net income	$ 299,377	$ 204,902	$ 172,852
Net income per common share
Basic	$ 8.01	$ 5.53	$ 4.68
Diluted	7.85	5.40	4.57
Cash dividends declared per common share	$ 2.40	$ 2.04	$ 1.68
Weighted average shares outstanding
Basic	37,359	37,083	36,958
Diluted	38,144	37,956	37,855
Commissions [Member]
Revenues
Revenues	$ 634,445	$ 463,856	$ 390,834
Information Services [Member]
Revenues
Revenues	34,341	30,730	28,227
Post-trade Services [Member]
Revenues
Revenues	19,460	15,763	15,346
Other [Member]
Revenues
Revenues	$ 879	$ 1,003	$ 1,158