Accrued Workers' Compensation Cost - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Total accrued costs, beginning of period	$ 236	$ 250	$ 238	$ 255
Incurred
Current year	16	19	35	39
Prior years	(11)	(6)	(16)	(13)
Total incurred	5	13	19	26
Paid
Current year	(2)	(2)	(3)	(2)
Prior years	(15)	(17)	(30)	(35)
Total paid	(17)	(19)	(33)	(37)
Total accrued costs, end of period	$ 224	$ 244	$ 224	$ 244