CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Operating activities
Net income	$ 163	$ 112
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	36	26
Stock-based compensation	31	21
Changes in operating assets and liabilities:
Prepaid income taxes	1	42
Prepaid expenses and other current assets	(24)	(1)
Workers' compensation collateral receivable and other noncurrent assets	(10)	(7)
Accounts payable and other current liabilities	(9)	7
Accrued corporate wages	(4)	1
Workers' compensation loss reserves and other noncurrent liabilities	0	4
Worksite employee related assets	(51)	(5)
Worksite employee related liabilities	(609)	(341)
Net cash used in operating activities	(476)	(141)
Investing activities
Purchases of marketable securities	(223)	0
Proceeds from sale of marketable securities	54	0
Proceeds from maturity of marketable securities	33	14
Acquisitions of property and equipment	(33)	(29)
Net cash used in investing activities	(169)	(15)
Financing activities
Repurchase of common stock	(47)	(39)
Proceeds from issuance of common stock on exercised options	6	9
Proceeds from issuance of common stock on employee stock purchase plan	3	2
Awards effectively repurchased for required employee withholding taxes	(15)	(8)
Proceeds from issuance of notes payable, net	210	0
Payments for extinguishment of debt	(204)	0
Repayment of notes payable	(15)	(29)
Net cash used in financing activities	(62)	(65)
Net decrease in cash and cash equivalents, unrestricted and restricted	(707)	(221)
Cash and cash equivalents, unrestricted and restricted:
Beginning of period	1,738	1,233
End of period	1,031	1,012
Supplemental disclosures of cash flow information
Interest paid	13	12
Income taxes paid, net	33	0
Supplemental schedule of noncash investing and financing activities
Payable for purchase of property and equipment	2	2
Supplemental schedule of cash and cash equivalents
Net increase (decrease) in unrestricted cash and cash equivalents	(99)	80
Net decrease in restricted cash and cash equivalents	$ 608	$ 301