Workers' Compensation Loss Reserves - Summary of Workers' Compensation Liabilities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Total loss reserves, end of period	$ 244	$ 250	$ 255	$ 255	$ 260	$ 255
Collateral paid to carriers and offset against loss reserves	(20)		(23)
Total loss reserves, net of carrier collateral offset	224		232
Payable in less than 1 year (net of collateral paid to carriers of $5 and $6 at June 30, 2018 and December 31, 2017, respectively)	67		67
Payable in more than 1 year (net of collateral paid to carriers of $15 and $17 at June 30, 2018 and December 31, 2017, respectively)	157		165
Collateral paid to insurance carriers, current	5		6
Collateral paid to insurance carriers, noncurrent	$ 15		$ 17