Accrued Workers' Compensation Costs - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Total accrued costs, beginning of period	$ 205	$ 214
Incurred
Current year	16	20
Prior years	(4)	(3)
Total incurred	12	17
Paid
Current year	(1)	(1)
Prior years	(11)	(13)
Total paid	(12)	(14)
Total accrued costs, end of period	$ 205	$ 217