CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Operating activities
Net income	$ 375	$ 355	$ 338
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization of intangible assets	72	64	54
Amortization of deferred costs	40	38	31
Amortization of ROU asset, lease modification, impairment, and abandonment	9	25	12
Stock based compensation	59	62	50
Accretion of discount rate on lease liabilities	2	2	2
Provision for doubtful accounts	3	2	0
Deferred income taxes	5	(22)	(9)
Losses from disposition of assets	1	6	0
Losses and impairment on investments	1	18	0
Changes in operating assets and liabilities:
Accounts receivable, net	(2)	0	3
Unbilled revenue, net	(72)	(51)	(78)
Prepaid expenses, net	4	(2)	(5)
Other payroll assets	(259)	(72)	10
Accounts payable and other current liabilities	(8)	(13)	33
Client deposits and other client liabilities	(40)	9	(37)
Accrued wages	77	65	60
Accrued health insurance costs, net	1	0	2
Accrued workers' compensation costs, net	(12)	(8)	(7)
Payroll taxes payable and other payroll withholdings	351	158	(166)
Operating lease liabilities	(17)	(17)	(13)
Other assets	(38)	(55)	(60)
Other liabilities	(7)	(2)	(2)
Net cash provided by operating activities	545	562	218
Investing activities
Purchases of marketable securities	(276)	(410)	(444)
Proceeds from sale and maturity of marketable securities	286	469	349
Acquisitions of property and equipment and projects in process	(75)	(56)	(40)
Acquisitions of subsidiaries, net of cash acquired	0	(229)	0
Other Investments	(5)	0	0
Net cash used in investing activities	(70)	(226)	(135)
Financing activities
Repurchase of common stock	(1,122)	(523)	(94)
Proceeds from issuance of common stock	15	11	11
Payment of long-term financing costs and debt issuance costs	(9)	0	(9)
Repayment of borrowings	0	0	(370)
Proceeds from revolving credit agreement borrowings	695	0	0
Repayment of borrowings under revolving credit agreement	(495)	0	0
Awards effectively repurchased for required employee withholding taxes	(30)	(24)	(26)
Net cash provided by (used in) financing activities	(546)	(536)	12
Effect of exchange rate changes on cash and cash equivalents	0	(1)	0
Net increase (decrease) in cash and cash equivalents, unrestricted and restricted	(71)	(201)	95
Cash and cash equivalents, unrestricted and restricted:
Beginning of period	1,537	1,738	1,643
End of period	1,466	1,537	1,738
Supplemental disclosures of cash flow information
Interest paid	25	18	12
Income taxes paid, net	114	128	129
Supplemental schedule of noncash investing and financing activities
Payable for purchase of property and equipment	4	6	3
Acquisitions of subsidiaries paid in stock	0	17	0
2031 Notes Payable
Financing activities
Proceeds from issuance of notes	400	0	0
2029 Notes Payable
Financing activities
Proceeds from issuance of notes	$ 0	$ 0	$ 500