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Derivative Instruments - Additional Information (Detail)
Mcf in Millions, $ in Millions
2 Months Ended 6 Months Ended 12 Months Ended
Feb. 29, 2016
Jun. 30, 2016
USD ($)
Mcf
Jun. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Mcf
Derivative [Line Items]        
Cash payment for settled interest-rate swap   $ 727 $ 77  
Debt instrument, principal amount   17,313   $ 17,312
Total Amount of Senior Notes Due in 2019 [Member]        
Derivative [Line Items]        
Debt instrument, principal amount   $ 900    
Zero-Coupon Senior Notes [Member]        
Derivative [Line Items]        
Debt instrument, maturity date   Oct. 10, 2036    
Debt Instrument, earliest call date   Oct. 10, 2016    
Zero-Coupon Senior Notes [Member] | Accreted Value at Next Potential Put Date [Member]        
Derivative [Line Items]        
Debt instrument, accreted value   $ 839    
Not Designated as Hedging Instrument [Member]        
Derivative [Line Items]        
Gross derivative liabilities   1,922   1,722
Amount of cash posted as collateral   599   58
Aggregate fair value of derivative instruments with credit-risk-related contingent features for which a net liability position existed (net of collateral)   1,300   1,300
Not Designated as Hedging Instrument [Member] | Settled Interest Rate Swap [Member]        
Derivative [Line Items]        
Cash payment for settled interest-rate swap   193    
Not Designated as Hedging Instrument [Member] | Interest-Rate Swaps 3 and 5 [Member] | Modified Interest-Rate Swaps for Mandatory Termination Dates [Member]        
Derivative [Line Items]        
Mandatory termination date for interest-rate swap prior to modification Sep. 15, 2021      
Notional principal amount of interest-rate swap   500    
Not Designated as Hedging Instrument [Member] | Interest-Rate Swaps 1 Through 7 [Member]        
Derivative [Line Items]        
Notional principal amount of interest-rate swap   1,700    
Not Designated as Hedging Instrument [Member] | Permitted to Offset Gross Derivative Asset with Financial Institutions [Member]        
Derivative [Line Items]        
Gross derivative liabilities   $ 100   $ 347
Natural Gas [Member] | Commodity [Member] | Not Designated as Hedging Instrument [Member]        
Derivative [Line Items]        
Financial derivative transactions | Mcf   5   8