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Financial Risk Management Activities - Gross and Net Fair Values (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Derivatives Fair Value [Line Items]    
Assets - Gross fair value of derivative contracts $ 13 $ 1
Assets - Net Fair Value of Derivative Contracts 13 1
Liabilities - Gross fair value of derivative contracts (4) (13)
Liabilities - Net Fair Value of Derivative Contracts (4) (13)
Derivative Contracts Designated as Hedging Instruments    
Derivatives Fair Value [Line Items]    
Assets - Gross fair value of derivative contracts 10  
Liabilities - Gross fair value of derivative contracts (4) (11)
Derivative Contracts Designated as Hedging Instruments | Commodity | Accounts Receivable    
Derivatives Fair Value [Line Items]    
Assets - Gross fair value of derivative contracts 10  
Derivative Contracts Designated as Hedging Instruments | Commodity | Accounts Payable    
Derivatives Fair Value [Line Items]    
Liabilities - Gross fair value of derivative contracts   (7)
Derivative Contracts Designated as Hedging Instruments | Interest Rate | Other Liabilities and Deferred Credits    
Derivatives Fair Value [Line Items]    
Liabilities - Gross fair value of derivative contracts (4)  
Derivative Contracts Designated as Hedging Instruments | Interest Rate | Accounts Payable    
Derivatives Fair Value [Line Items]    
Liabilities - Gross fair value of derivative contracts   (4)
Derivative Contracts Not Designated as Hedging Instruments    
Derivatives Fair Value [Line Items]    
Assets - Gross fair value of derivative contracts 3 1
Liabilities - Gross fair value of derivative contracts   (2)
Derivative Contracts Not Designated as Hedging Instruments | Commodity | Accounts Receivable    
Derivatives Fair Value [Line Items]    
Assets - Gross fair value of derivative contracts $ 3  
Derivative Contracts Not Designated as Hedging Instruments | Commodity | Accounts Payable    
Derivatives Fair Value [Line Items]    
Liabilities - Gross fair value of derivative contracts   (2)
Derivative Contracts Not Designated as Hedging Instruments | Foreign Exchange    
Derivatives Fair Value [Line Items]    
Assets - Gross fair value of derivative contracts   $ 1