CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Worksite employee related assets:
Advance collections	$ 14	$ 12
Worksite employee related liabilities:
Prepayments	38	19
Collateral paid, current	5	6
Collateral paid, noncurrent	$ 15	$ 17
Preferred stock ($0.000025 par value per share; 20,000,000 shares authorized; no shares issued and outstanding at June 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; 70,573,887 and 69,818,392 shares issued and outstanding at June 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)	70,573,887	69,818,392
Common stock, shares outstanding (in shares)	70,573,887	69,818,392