Workers' Compensation Loss Reserves - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Total loss reserves, beginning of period	$ 244	$ 255	$ 255	$ 255
Incurred
Current year	20	22	59	70
Prior years	(4)	(4)	(17)	(3)
Total incurred	16	18	42	67
Paid
Current year	(6)	(4)	(8)	(9)
Prior years	(9)	(20)	(44)	(64)
Total paid	(15)	(24)	(52)	(73)
Total loss reserves, end of period	$ 245	$ 249	$ 245	$ 249