Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues:
Total revenues	$ 247,287	$ 240,135	$ 702,866	$ 675,706
Cost of revenues:
Total cost of revenues	187,602	185,218	578,437	566,125
Gross margin	59,685	54,917	124,429	109,581
Selling, general and administrative expenses	36,670	33,925	101,713	88,595
Depreciation and amortization	949	1,062	3,228	2,989
Income from operations	22,066	19,930	19,488	17,997
Other income (expense):
Investment income, net	2,235	1,534	6,455	3,084
Interest expense	(465)	(52)	(574)	(197)
Other, net	10	32	15	28
Other income, net	1,780	1,514	5,896	2,915
Income before income taxes	23,846	21,444	25,384	20,912
Provision for income taxes	4,759	6,659	4,178	6,228
Net income	$ 19,087	$ 14,785	$ 21,206	$ 14,684
Basic income per common share	$ 2.59	$ 2.03	$ 2.89	$ 2.02
Weighted average number of basic common shares outstanding	7,369	7,296	7,327	7,266
Diluted income per common share	$ 2.50	$ 1.96	$ 2.77	$ 1.95
Weighted average number of diluted common shares outstanding	7,643	7,527	7,652	7,539
Cash dividends per common share	$ 0.25	$ 0.25	$ 0.75	$ 0.75
Professional Employer Service Fees [Member]
Revenues:
Total revenues	$ 206,320	$ 197,388	$ 592,559	$ 557,315
Staffing Services [Member]
Revenues:
Total revenues	40,967	42,747	110,307	118,391
Direct Payroll Costs [Member]
Cost of revenues:
Total cost of revenues	30,842	31,986	83,265	89,182
Payroll Taxes and Benefits [Member]
Cost of revenues:
Total cost of revenues	100,348	94,922	322,784	304,268
Workers' Compensation [Member]
Cost of revenues:
Total cost of revenues	$ 56,412	$ 58,310	$ 172,388	$ 172,675