Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Revenues	$ 435,565	$ 393,422	$ 367,730
Expenses
Employee compensation and benefits	109,117	102,313	96,625
Depreciation and amortization	23,080	19,274	17,838
Technology and communications	23,866	20,048	17,275
Professional and consulting fees	21,521	19,367	17,175
Occupancy	14,176	6,125	4,681
Marketing and advertising	12,114	9,762	8,934
Clearing costs	7,754	5,797	6,060
General and administrative	11,353	11,121	9,157
Total expenses	222,981	193,807	177,745
Operating income	212,584	199,615	189,985
Other income (expense)
Investment income	6,112	3,619	2,137
Other, net	(610)	(1,466)	(520)
Total other income	5,502	2,153	1,617
Income before income taxes	218,086	201,768	191,602
Provision for income taxes	45,234	53,679	65,430
Net income	$ 172,852	$ 148,089	$ 126,172
Net income per common share
Basic	$ 4.68	$ 4.02	$ 3.42
Diluted	4.57	3.89	3.34
Cash dividends declared per common share	$ 1.68	$ 1.32	$ 1.04
Weighted average shares outstanding
Basic	36,958	36,864	36,844
Diluted	37,855	38,038	37,738
Commissions [Member]
Revenues
Revenues	$ 390,834	$ 355,282	$ 332,307
Information Services [Member]
Revenues
Revenues	28,227	25,806	23,269
Post-trade Services [Member]
Revenues
Revenues	15,346	11,090	10,812
Other [Member]
Revenues
Revenues	$ 1,158	$ 1,244	$ 1,342