Description of Business and Significant Accounting Policies - Effects on cash flow due to new accounting pronouncements (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Operating activities
Accounts receivable	$ 3	$ 11
Unbilled revenue	(36)	35
Prepaid income taxes		0
Prepaid expenses and other current assets		0
Prepaid expenses	(18)	(9)
Workers' compensation collateral receivable		0
Accounts payable and other current liabilities	(11)	(28)
Client deposits	10	(24)
Accrued wages	25	(28)
Accrued corporate wages		0
Accrued health insurance costs	(13)	(13)
Accrued workers' compensation costs	9	(8)
Workers' compensation loss reserves and other non-current liabilities		0
Payroll taxes payable and other payroll withholdings		(588)
Worksite employee related assets		0
Worksite employee related liabilities		0
Other assets	(30)	(45)
Other liabilities	$ (2)	(1)
As previously reported
Operating activities
Accounts receivable		0
Unbilled revenue		0
Prepaid income taxes		2
Prepaid expenses and other current assets		(23)
Prepaid expenses		0
Workers' compensation collateral receivable		(6)
Accounts payable and other current liabilities		(14)
Client deposits		0
Accrued wages		0
Accrued corporate wages		(5)
Accrued health insurance costs		0
Accrued workers' compensation costs		0
Workers' compensation loss reserves and other non-current liabilities		(4)
Payroll taxes payable and other payroll withholdings		0
Worksite employee related assets		4
Worksite employee related liabilities		(652)
Other assets		0
Other liabilities		0
Reclassified amounts
Operating activities
Accounts receivable		11
Unbilled revenue		35
Prepaid income taxes		(2)
Prepaid expenses and other current assets		23
Prepaid expenses		(9)
Workers' compensation collateral receivable		6
Accounts payable and other current liabilities		(14)
Client deposits		(24)
Accrued wages		(28)
Accrued corporate wages		5
Accrued health insurance costs		(13)
Accrued workers' compensation costs		(8)
Workers' compensation loss reserves and other non-current liabilities		4
Payroll taxes payable and other payroll withholdings		(588)
Worksite employee related assets		(4)
Worksite employee related liabilities		652
Other assets		(45)
Other liabilities		$ (1)