Workers' Compensation Loss Reserves - Summary of Workers' Compensation Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Total loss reserves, end of period	$ 250	$ 255	$ 261	$ 255
Collateral paid to carriers and offset against loss reserves	(21)	(23)
Total loss reserves, net of carrier collateral offset	229	232
Payable in less than 1 year (net of collateral paid to carriers of $5 and $6 at March 31, 2018 and December 31, 2017, respectively)	67	67
Payable in more than 1 year (net of collateral paid to carriers of $16 and $17 at March 31, 2018 and December 31, 2017, respectively)	162	165
Collateral paid to carriers, current	5	6
Collateral paid to carriers, noncurrent	$ 16	$ 17