Other Balance Sheet Information (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 577,070	$ 678,512
Short-term Investments	116,670	90,031
Cash	14,523	41,047
Drafts Payable	(15,838)	(43,694)
Cash, Cash Equivalents, and Short-term Investments	692,425	765,896
Client Prepayments Included in Cash Balance	(20,200)	(36,800)
Payroll Withholdings Included in Cash Balance	482,700	504,800
Supplemental cash flow information:
Restricted cash	51,263	49,779	$ 51,829	$ 46,929
Funds Held for Clients	29,835	34,942	25,277	31,732
Deposits workers compensation	197,466	196,370	179,014	185,027
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	957,152	1,013,919	$ 818,263	$ 839,500
PayrollTaxesAndOtherPayrollDeductionsPayable [Abstract]
Payroll taxes and other payroll deductions payable	518,061	556,085
ERC Funds Received Included in Cash Balance	149,300
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	13,837	33,068
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	13,837	33,068
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	577,070	678,512
Short-term Investments	102,833	56,963
Cash	14,523	41,047
Drafts Payable	(15,838)	(43,694)
Cash, Cash Equivalents, and Short-term Investments	$ 678,588	$ 732,828