CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Advance collections	$ 39	$ 12
Health benefit loss reserves	45	0
Worksite employee related liabilities:
Prepayments	0	19
Collateral paid	4	6
Workers' compensation, collateral paid	$ 14	$ 17
Preferred stock ($0.000025 par value per share; 20,000,000 shares authorized; no shares issued and outstanding at September 30, 2018 and December 31, 2017)
Preferred stock, par value (in dollars per share)	$ 0.000025	$ 0.000025
Preferred stock, shares authorized (in shares)	20,000,000	20,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock and additional paid-in capital ($0.000025 par value per share; 750,000,000 shares authorized; 70,508,389 and 69,818,392 shares issued and outstanding at September 30, 2018 and December 31, 2017, respectively)
Common stock, par value (in dollars per share)	$ 0.000025	$ 0.000025
Common stock, shares authorized (in shares)	750,000,000	750,000,000
Common stock, shares issued (in shares)	70,508,389	69,818,392
Common stock, shares outstanding (in shares)	70,508,389	69,818,392