Worksite Employee Related Assets and Liabilities - WSE Related Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Worksite employee related assets:
Restricted cash, cash equivalents and investments	$ 170	$ 131
Payroll funds collected	1,095	826
Prepaid insurance premiums	11	9
Workers' compensation collateral receivable	39	32
Total worksite employee related assets	1,625	1,281
Worksite employee related liabilities:
Total worksite employee related liabilities	1,618	1,276
WSE
Worksite employee related assets:
Restricted cash, cash equivalents and investments	170	131
Payroll funds collected	1,095	826
Unbilled revenues (net of advance collections of $12 and $9 at December 31, 2017 and 2016, respectively)	297	293
Accounts receivable	20	5
Prepaid insurance premiums	25	13
Workers' compensation collateral receivable	1	2
Other payroll assets	17	11
Total worksite employee related assets	1,625	1,281
Advance collection	12	9
Worksite employee related liabilities:
Accrued wages	289	273
Client deposits	52	56
Payroll tax liabilities	981	692
Health benefits loss reserves (net of prepayments of $19 and $0 at December 31, 2017 and 2016, respectively)	151	129
Workers' compensation loss reserves (net of collateral paid of $6 and $10 at December 31, 2017 and 2016, respectively)	67	64
Insurance premiums and other payables	25	14
Other payroll withholdings	53	48
Total worksite employee related liabilities	1,618	1,276
Prepayments	19	0
Collateral paid	$ 6	$ 10