Other Balance Sheet Information (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Cash and Cash Equivalents [Line Items]
Overnight Holdings	$ 635,931	$ 678,512
Short-term Investments	91,081	90,031
Cash	21,072	41,047
Drafts Payable	(15,961)	(43,694)
Cash, Cash Equivalents, and Short-term Investments	732,123	765,896
Client Prepayments Included in Cash Balance	(42,900)	(36,800)
Payroll Withholdings Included in Cash Balance	457,800	504,800
Supplemental Cash Flow Elements [Abstract]
Restricted cash	48,113	49,779	$ 49,429	$ 46,929
Funds Held for Clients	41,552	34,942	0	0
Deposits workers compensation	211,226	196,370	190,756	185,027
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	997,479	1,013,919	$ 816,839	$ 807,768
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	0	0
Short-term Investments	35,535	33,068
Cash	0	0
Drafts Payable	0	0
Cash, Cash Equivalents, and Short-term Investments	35,535	33,068
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings	635,931	678,512
Short-term Investments	55,546	56,963
Cash	21,072	41,047
Drafts Payable	(15,961)	(43,694)
Cash, Cash Equivalents, and Short-term Investments	$ 696,588	$ 732,828