Accrued Workers' Compensation Costs - Summary of Workers' Compensation Liabilities (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Insurance [Abstract]
Total accrued costs, end of period	$ 198	$ 198	$ 205	$ 210	$ 211	$ 214
Collateral paid to carriers and offset against accrued costs	(7)		(8)
Total accrued costs, net of carrier collateral offset	191		197
Payable in less than 1 year (net of collateral paid to carriers)	54		59
Payable in more than 1 year (net of collateral paid to carriers)	137		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	$ 5		$ 5