CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Cash flows from operating activities
Net income	$ 69,884	$ 61,922
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	10,184	8,047
Stock-based compensation	9,846	11,822
Deferred income taxes	10,006	22,946
Changes in operating assets and liabilities:
Accounts receivable	(156,917)	(168,498)
Prepaid insurance	(11,431)	(47,506)
Other current assets	(26,970)	(14,257)
Other assets and ROU assets	1,588	(1,508)
Accounts payable	1,414	(362)
Payroll taxes and other payroll deductions payable	(83,125)	(67,441)
Accrued worksite employee payroll expense	235,970	168,498
Accrued health insurance costs	11,398	39,451
Accrued workers’ compensation costs	(7,050)	(3,393)
Accrued corporate payroll, commissions and other accrued liabilities	(18,971)	(5,421)
Income taxes payable/receivable	16,197	(5,345)
Total adjustments	(11,037)	(59,951)
Net cash provided by operating activities	58,847	1,971
Marketable securities:
Purchases	(6,964)	(10,585)
Proceeds from maturities	5,760	10,580
Property and equipment:
Purchases	(4,686)	(12,072)
Net cash used in investing activities	(5,890)	(12,077)
Cash flows from financing activities:
Purchase of treasury stock	(27,441)	(29,686)
Dividends paid	(17,244)	(15,461)
Other	(799)	(2,751)
Net cash used in financing activities	(43,886)	(42,396)
Net increase (decrease) in cash, cash equivalents and restricted cash	9,071	(52,502)
Cash, cash equivalents and restricted cash beginning of period	807,768	786,699
Cash, cash equivalents and restricted cash end of period	816,839	734,197
Supplemental Cash Flow Information [Abstract]
Cash and cash equivalents	576,654	494,777
Restricted cash	49,429	46,353
Deposits workers compensation	190,756	193,067
ROU assets obtained in exchange for lease obligations	$ 775	$ 12,104