Income Taxes (Schedule of Components of Deferred Income Taxes) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Net operating loss and credit carryforwards	$ 583	$ 412
Self-insurance liabilities	78	130
Long-term debt and interest	6
Accounts receivable	144	70
Accrued vacation	27	56
Other comprehensive income	9	38
Stock-based compensation	10	23
Deferred compensation	77	132
Other	89	125
Total Deferred Income Tax Assets, Gross	1,023	986
Valuation Allowance	(489)	(385)
Total Deferred Income Tax Assets, Net	534	601
Liabilities
Property and equipment	263	583
Prepaid expenses	30	61
Intangibles	128	238
Investments in unconsolidated affiliates	54	81
Other liabilities	16	22
Long-term debt and interest		12
Total Deferred Income Tax Liabilities, Gross	$ 491	$ 997