Accrued Workers' Compensation Costs - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Total accrued costs, beginning of period	$ 198	$ 211	$ 205	$ 214
Incurred
Current year	16	15	49	48
Prior years	(3)	(3)	(18)	(12)
Total incurred	13	12	31	36
Paid
Current year	(3)	(2)	(6)	(5)
Prior years	(10)	(11)	(32)	(35)
Total paid	(13)	(13)	(38)	(40)
Total accrued costs, end of period	$ 198	$ 210	$ 198	$ 210