CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating activities
Net income	$ 101	$ 91
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	19	15
Amortization of ROU	3	4
Accretion of discount rate on lease liabilities	1	0
Amortization of premium of investments	1	0
Stock based compensation	11	9
Changes in operating assets and liabilities:
Accounts receivable, net	13	0
Unbilled revenue, net	(94)	(95)
Prepaid expenses, net	5	(3)
Accounts payable and other current liabilities	33	29
Client deposits and other client liabilities	17	(6)
Accrued wages	179	38
Accrued health insurance costs, net	(7)	(2)
Accrued workers' compensation costs, net	0	3
Payroll taxes payable and other payroll withholdings	(426)	(334)
Operating lease liabilities	(3)	(5)
Other assets	(24)	(28)
Other liabilities	(4)	2
Net cash used in operating activities	(175)	(282)
Investing activities
Purchases of marketable securities	(82)	(155)
Proceeds from sale and maturity of marketable securities	84	67
Acquisitions of property and equipment	(5)	(6)
Net cash used in investing activities	(3)	(94)
Financing activities
Repurchase of common stock	(60)	(40)
Awards effectively repurchased for required employee withholding taxes	(5)	(3)
Proceeds from revolving credit agreement borrowings	0	234
Payment of long-term financing fees	(2)	0
Payment of debt issuance costs	(7)	0
Proceeds from issuance of 2029 Notes	500	0
Repayment of debt	(370)	(6)
Net cash provided by financing activities	56	185
Net decrease in cash and cash equivalents, unrestricted and restricted	(122)	(191)
Cash and cash equivalents, unrestricted and restricted:
Beginning of period	1,643	1,456
End of period	1,521	1,265
Supplemental disclosures of cash flow information
Interest paid	2	3
Income taxes paid, net	2	1
Supplemental schedule of noncash investing and financing activities
Payable for purchase of property and equipment	$ 3	$ 1