Accrued Workers' Compensation Cost - Additional Information (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Financial Guarantee Insurance Contracts, Claim Liability [Line Items]
Collateral held by insurance carriers	$ 56	$ 57
Liability For Unpaid Claims And Claims Adjustment Expense, Collateral	$ 13	13
Collateral for workers' compensation claims
Financial Guarantee Insurance Contracts, Claim Liability [Line Items]
Liability For Unpaid Claims And Claims Adjustment Expense, Collateral		$ 13