Other Balance Sheet Information (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	$ 678,512	$ 490,154
Short-term Investments	90,031	120,742
Cash	41,047	47,331
Drafts Payable	(43,694)	(50,624)
Cash, Cash Equivalents, and Short-term Investments	765,896	607,603
Client Prepayments Included in Cash Balance	36,800	20,054
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance	504,800	424,800
Restricted Cash and Cash Equivalents, Current	49,779	46,929	$ 45,522	$ 49,295
Cash and cash equivalents	732,828	575,812	554,846	367,342
Funds Held for Clients	34,942		0	0
Deposits workers' compensation	196,370	185,027	186,331	175,913
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	1,013,919		$ 786,699	$ 592,550
Reclassification, Other [Member]
Funds Held for Clients		31,732
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		839,500
Previously Reported [Member]
Funds Held for Clients		0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		807,768
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	0	0
Short-term Investments	33,068	31,791
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	33,068	31,791
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Money Market Funds, Overnight Holdings, at Carrying Value	678,512	490,154
Short-term Investments	56,963	88,951
Cash	41,047	47,331
Drafts Payable	(43,694)	(50,624)
Cash, Cash Equivalents, and Short-term Investments	$ 732,828	$ 575,812