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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Long-term Debt, Fair Value $ 370 $ 370 $ 370
Nonrecurring fair value measurements [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Impaired fixed assets $ 17