Fair Value Measurements (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets and Liabilities Measured on Recurring Basis Disclosure Items [Line Items]
Fair value amount of Level 1 into Level 2 (Asset)	$ 0
Fair value amount of Level 2 into Level 1 (Asset)	0
Fair value amount of Level 1 into Level 2 (Liability)	0
Fair value amount of Level 2 into Level 1 (Liability)	0
Carrying Amount [Member]
Fair Value Assets and Liabilities Measured on Recurring Basis Disclosure Items [Line Items]
Long-term debt, including current portion	4,753,000,000	$ 4,865,000,000
Fair Value [Member]
Fair Value Assets and Liabilities Measured on Recurring Basis Disclosure Items [Line Items]
Long-term debt, including current portion	5,034,000,000	5,079,000,000
Fair Value, Inputs, Level 1 [Member]
Fair Value Assets and Liabilities Measured on Recurring Basis Disclosure Items [Line Items]
Investments, including money market funds	51,000,000	59,000,000
Fair Value, Inputs, Level 2 [Member]
Fair Value Assets and Liabilities Measured on Recurring Basis Disclosure Items [Line Items]
Assets or liabilities	0	0
Fair Value, Inputs, Level 3 [Member]
Fair Value Assets and Liabilities Measured on Recurring Basis Disclosure Items [Line Items]
Assets or liabilities	$ 0	$ 0