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Fair Value Measurements (Schedule of Carrying Values and Fair Values of Financial Instruments) (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cost investments, carrying amount $ 165 $ 159
Cost investments, fair value 0 0
Insurance contracts in nonqualified trusts, carrying amount 40 42
Insurance contracts in nonqualified trusts, fair value 40 42
Long-term debt, including current installments of long-term debt, carrying amount 3,286 3,398
Long-term debt, including current installments of long-term debt, fair value 3,344 3,507
Significant Other Observable Inputs (Level 2) [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cost investments, fair value 0 0
Insurance contracts in nonqualified trusts, fair value 40 42
Long-term debt, including current installments of long-term debt, fair value 3,171 3,299
Unobservable Inputs (Level 3) [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Cost investments, fair value 0 0
Insurance contracts in nonqualified trusts, fair value 0 0
Long-term debt, including current installments of long-term debt, fair value $ 173 $ 208