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Schedule of Valuation Processes (Detail) - Level 3
$ in Millions
Sep. 30, 2020
USD ($)
$ / gal
Mar. 31, 2020
USD ($)
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Assets | $   $ 352
Market Approach Valuation Technique | Derivative Liabilities | Natural Gas Liquids    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Liabilities, fair value | $ $ (2)  
Market Approach Valuation Technique | Derivative Liabilities | Natural Gas Liquids | Minimum    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Forward curve prices 0.14  
Market Approach Valuation Technique | Derivative Liabilities | Natural Gas Liquids | Maximum    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Forward curve prices 0.89  
Market Approach Valuation Technique | Derivative Liabilities | Natural Gas Liquids | Weighted Average [Member]    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Forward curve prices 0.43  
Market Approach Valuation Technique | Derivative Assets | Natural Gas [Member]    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Assets | $ $ 4  
Market Approach Valuation Technique | Derivative Assets | Natural Gas [Member] | Minimum    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Forward curve prices 1.90  
Market Approach Valuation Technique | Derivative Assets | Natural Gas [Member] | Maximum    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Forward curve prices 2.68  
Market Approach Valuation Technique | Derivative Assets | Natural Gas [Member] | Weighted Average [Member]    
Fair Value Inputs Assets And Liabilities Quantitative Information [Line Items]    
Forward curve prices 1.86