Other Balance Sheet Information (Details) - USD ($) $ in Thousands		Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Cash and Cash Equivalents [Line Items]
Overnight Holdings		$ 503,228	$ 490,154
Short-term Investments		77,982	120,742
Cash		31,111	47,331
Drafts Payable		(15,685)	(50,624)
Cash, Cash Equivalents, and Short-term Investments		596,636	607,603
Client Prepayments Included in Cash Balance		(35,800)	(20,100)
Payroll Withholdings Included in Cash Balance		321,900	424,800
Supplemental Cash Flow Elements [Abstract]
Funds Held for Clients	[1]	25,277	31,732	$ 0	$ 0
Deposits workers compensation		179,014	185,027	208,236	186,331
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		818,263	839,500	$ 723,970	$ 786,699
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings		0	0
Short-term Investments		34,493	31,791
Cash		0	0
Drafts Payable		0	0
Cash, Cash Equivalents, and Short-term Investments		34,493	31,791
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Overnight Holdings		503,228	490,154
Short-term Investments		43,489	88,951
Cash		31,111	47,331
Drafts Payable		(15,685)	(50,624)
Cash, Cash Equivalents, and Short-term Investments		$ 562,143	$ 575,812

[1] Beginning in the third quarter of 2022, we adjusted the presentation of our Consolidated Statements of Cash Flows to include changes in funds held for clients as a financing activity and to include funds held for clients in both the beginning and ending period amounts in our totals of cash, cash equivalents, restricted cash and funds held for clients. Prior period amounts have not been adjusted to this presentation as the amounts are immaterial to our consolidated financial statements. Previously, the changes in funds held for clients and the related client fund liabilities were presented within operating activities in our Consolidated Statements of Cash Flows. Funds held for clients are held in a trust separate from our company funds and we do not use these funds held for clients for any corporate activity