Accrued Workers' Compensation Costs - Summary of Workers' Compensation Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Total accrued costs, end of year	$ 214	$ 238	$ 255	$ 255
Collateral paid to carriers and offset against accrued costs	(9)	(13)
Total loss reserves, net of carrier collateral offset	205	225
Accrued workers' compensation costs, net	61	67
Payable in more than 1 year	144	158
Collateral paid, current	3	3
Collateral paid, noncurrent	6	10
WSE
Liability for Claims and Claims Adjustment Expense [Line Items]
Total loss reserves, net of carrier collateral offset	205	225
Accrued workers' compensation costs, net	61	67
Payable in more than 1 year	$ 144	$ 158