Workers' Compensation Loss Reserves - Summary of Workers' Compensation Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Insurance [Abstract]
Total loss reserves, end of period	$ 245	$ 244	$ 255	$ 249	$ 255	$ 255
Collateral paid to carriers and offset against loss reserves	(18)		(23)
Total loss reserves, net of carrier collateral offset	227		232
Payable in less than 1 year (net of collateral paid to carriers of $4 and $6 at September 30, 2018 and December 31, 2017, respectively)	68		67
Payable in more than 1 year (net of collateral paid to carriers of $14 and $17 at September 30, 2018 and December 31, 2017, respectively)	159		165
Collateral paid to insurance carriers, current	4		6
Collateral paid to insurance carriers, noncurrent	$ 14		$ 17