Workers' Compensation Loss Reserves - Summary of Workers' Compensation Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Total loss reserves, end of year	$ 255	$ 255	$ 190	$ 148
Collateral paid to carriers and offset against loss reserves	(23)	(32)
Total loss reserves, net of carrier collateral offset	232	223
Payable in more than 1 year	165	159
Collateral paid to carriers, noncurrent	17	22
WSE
Liability for Claims and Claims Adjustment Expense [Line Items]
Total loss reserves, net of carrier collateral offset	232	223
Payable in less than 1 year	67	64
Payable in more than 1 year	165	159
Collateral paid to carriers, current	6	10
Collateral paid to carriers, noncurrent	$ 17	$ 22