Accrued Workers' Compensation Cost - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Total accrued costs, beginning of period	$ 214	$ 238
Incurred
Current year	20	19
Prior years	(3)	(5)
Total incurred	17	14
Paid
Current year	(1)	(1)
Prior years	(13)	(15)
Total paid	(14)	(16)
Total accrued costs, end of period	$ 217	$ 236