Accrued Workers' Compensation Costs - Summary of Activities in Liability for Unpaid Claims and Claims Adjustment Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense
Total accrued costs, beginning of year	$ 238	$ 255	$ 255
Incurred
Current year	72	80	98
Prior years	(31)	(28)	(6)
Total incurred	41	52	92
Paid
Current year	(14)	(12)	(14)
Prior years	(51)	(57)	(78)
Total paid	(65)	(69)	(92)
Total accrued costs, end of year	$ 214	$ 238	$ 255