CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Assets
Cash and cash equivalents	$ 692,873	$ 732,828
Restricted cash	57,403	49,779
Marketable securities	15,905	33,068
Accounts receivable, net	693,878	622,764
Prepaid insurance and related assets	7,013	11,706
Other current assets	128,220	61,728
Total current assets	1,595,292	1,511,873
Property and equipment, net of accumulated depreciation	197,424	199,992
Right-of-use (“ROU”) leased assets	57,438	56,532
nsp:Depositsandprepaidhealthinsurance	215,070	213,270
Goodwill and other intangible assets, net	12,707	12,707
Deferred income taxes, net	20,347	15,533
Other assets	21,381	29,354
Total assets	2,119,659	2,039,261
Current liabilities:
Accounts payable	10,693	7,732
Payroll taxes and other payroll deductions payable	566,373	556,085
Accrued worksite employee payroll cost	559,194	513,397
Accrued health insurance costs	46,460	53,402
Accrued workers’ compensation costs	60,475	53,485
Accrued corporate payroll and commissions	64,286	89,147
Other accrued liabilities	128,808	80,122
Total current liabilities	1,436,289	1,353,370
Noncurrent liabilities:
Accrued workers’ compensation costs	162,852	179,629
Long-term debt	369,400	369,400
Operating Lease, Liability, Noncurrent	57,494	55,587
Total noncurrent liabilities	589,746	604,616
Commitments and contingencies
Stockholders’ equity:
Preferred Stock, Value, Issued	0	0
Common stock	555	555
Additional paid-in capital	$ 185,031	$ 151,144
Treasury Stock, Common, Shares	18,208,000	17,608,000
Treasury stock, at cost	$ (830,524)	$ (725,532)
Accumulated Other Comprehensive Income (Loss), Net of Tax	9	(82)
Retained earnings	738,553	655,190
Total stockholders’ equity	93,624	81,275
Total liabilities and stockholders’ equity	$ 2,119,659	$ 2,039,261
Stockholders' Equity, Number of Shares, Par Value and Other Disclosure [Abstract]
Common Stock, Par or Stated Value Per Share	$ 0.01	$ 0.01
Preferred Stock, Shares Authorized	20,000,000	20,000,000
Preferred Stock, Shares Issued	0	0
Common Stock, Shares Authorized	120,000,000	120,000,000
Common Stock, Shares, Issued	55,489,000	55,489,000
Common Stock, Shares, Outstanding	37,281,000	37,881,000
Treasury Stock, Common, Shares	18,208,000	17,608,000