Accrued Workers' Compensation Costs - Summary of Workers' Compensation Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Insurance [Abstract]
Total accrued costs, end of period	$ 205	$ 205	$ 217	$ 214
Collateral paid to carriers and offset against accrued costs	(8)	(8)
Total accrued costs, net of carrier collateral offset	197	197
Payable in less than 1 year (net of collateral paid to carriers)	59	59
Payable in more than 1 year (net of collateral paid to carriers)	138	138
Net of collateral paid to carriers, payable in less than one year	2	3
Net of collateral paid to carriers, payable in more than 1 year	$ 6	$ 5