Workers' Compensation Loss Reserves - Summary of Workers' Compensation Loss Reserve Activity (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Total loss reserves, beginning of period	$ 255	$ 255
Incurred
Current year	20	27
Prior years	(7)	1
Total incurred	13	28
Paid
Current year	0	0
Prior years	(18)	(22)
Total paid	(18)	(22)
Total loss reserves, end of period	$ 250	$ 261