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Long-Term and Short-Term Debt (Schedule of Line of Credit Facilities) (Detail) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
Current Joint Revolving Credit Facility    
Debt Instrument [Line Items]    
Facility limit $ 1,000,000,000  
Outstanding Commercial Paper 0  
Outstanding Letters of Credit $ 0  
Previous Joint Revolving Credit Facility    
Debt Instrument [Line Items]    
Facility limit [1]   $ 1,200,000,000
Outstanding Commercial Paper   73,000,000
Outstanding Letters of Credit   $ 0
[1] Included $500 million related to Fuel Company. In February 2019, Fuel Company's commercial paper program and its credit facility were terminated