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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, 2018
Dec. 31, 2017
Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense      
Total accrued costs, beginning of year $ 255 $ 255 $ 190
Incurred      
Current year 80 98 113
Prior years (28) (6) 28
Total incurred 52 92 141
Paid      
Current year (12) (14) (14)
Prior years (57) (78) (62)
Total paid (69) (92) (76)
Total accrued costs, end of year $ 238 $ 255 $ 255