Workers' Compensation Loss Reserves - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Total loss reserves, beginning of period	$ 250	$ 260	$ 255	$ 255
Incurred
Current year	19	20	39	48
Prior years	(6)	1	(13)	1
Total incurred	13	21	26	49
Paid
Current year	(2)	(5)	(2)	(6)
Prior years	(17)	(21)	(35)	(43)
Total paid	(19)	(26)	(37)	(49)
Total loss reserves, end of period	$ 244	$ 255	$ 244	$ 255