Workers' Compensation Loss Reserves - Summary of Activities in Liability for Unpaid Claims and Claims Adjustment Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense
Total loss reserves, beginning of year	$ 255	$ 190	$ 148
Incurred
Current year	98	113	89
Prior years	(6)	28	27
Total incurred	92	141	116
Paid
Current year	(14)	(14)	(16)
Prior years	(78)	(62)	(58)
Total paid	(92)	(76)	(74)
Total loss reserves, end of year	$ 255	$ 255	$ 190