CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Assets
Cash and cash equivalents	$ 575,812	$ 554,846
Restricted cash	46,929	45,522
Marketable securities	31,791	34,529
Accounts receivable, net	513,306	392,746
Prepaid insurance	11,285	10,164
Other current assets	53,312	39,461
Income taxes receivable	12,413	0
Total current assets	1,244,848	1,077,268
Property and equipment, net	210,723	216,256
Right-of-use “ROU” leased assets	62,830	60,663
Deposits and prepaid health insurance	201,927	203,231
Goodwill and other intangible assets, net	12,707	12,707
Deferred income taxes, net	4,892	9,603
Other assets	15,158	4,548
Total assets	1,753,085	1,584,276
Liabilities and stockholders’ equity
Accounts payable	6,412	6,203
Payroll taxes and other payroll deductions payable	467,892	377,960
Accrued worksite employee payroll cost	409,653	334,836
Accrued health insurance costs	50,001	32,685
Accrued workers’ compensation costs	50,534	48,186
Accrued corporate payroll and commissions	74,778	44,277
Other accrued liabilities	69,303	60,777
Total current liabilities	1,128,573	904,924
Accrued workers’ compensation costs, net of current	192,694	195,239
Long-term debt	369,400	369,400
Operating lease liabilities, net of current	64,192	64,289
Other accrued liabilities, net of current	0	6,292
Total noncurrent liabilities	626,286	635,220
Commitments and contingencies
Preferred stock, issued (in shares)	0	0
Preferred stock, par value (in dollars per share)	$ 0.01
Stockholders' Equity Attributable to Parent [Abstract]
Preferred stock ($0.01 per share par value; 20,000 shares authorized; no shares issued and outstanding)	$ 0	$ 0
Common stock ($0.01 per share par value; 120,000 shares authorized; 55,489 shares issued and outstanding)	555	555
Additional paid-in capital	109,179	95,528
Treasury stock, at cost (17,159 and 17,236 shares held in treasury)	(665,089)	(626,984)
Accumulated other comprehensive income (loss), net of tax	(9)	5
Retained earnings	553,590	575,028
Retained earnings	(1,774)	44,132
Total liabilities and stockholders’ equity	$ 1,753,085	$ 1,584,276
Stockholders' Equity, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock, authorized (in shares)	20,000,000
Common stock, par value (in dollars per share)	$ 0.01
Common stock, authorized (in shares)	120,000,000
Common stock, issued (in shares)	55,489,000	55,489,000
Treasury stock, shares (in shares)	17,159,000	17,236,000