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Derivative Instruments - Additional Information (Detail)
$ in Millions
2 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Feb. 29, 2016
Sep. 30, 2016
USD ($)
Mar. 31, 2016
USD ($)
Sep. 30, 2016
USD ($)
Bcf
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Bcf
Derivative [Line Items]            
Cash payment for settled interest-rate swap       $ 639 $ 44  
Debt instrument, principal amount   $ 17,285   17,285   $ 17,312
Return of collateral due to a negotiated increase of a credit threshold   200        
Amount of collateral posted related to derivative instruments with credit-risk-related contingent features   422   422   58
Total Amount of Senior Notes Due in 2019 [Member]            
Derivative [Line Items]            
Debt instrument, principal amount   900   $ 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   $ 839    
Not Designated as Hedging Instrument [Member]            
Derivative [Line Items]            
Gross derivative liabilities   1,904   1,904   1,722
Aggregate fair value of derivative instruments with credit-risk-related contingent features for which a net liability position existed (net of collateral)   1,400   1,400   1,300
Not Designated as Hedging Instrument [Member] | Settled Interest Rate Swap 1 [Member]            
Derivative [Line Items]            
Cash payment for settled interest-rate swap     $ 193      
Not Designated as Hedging Instrument [Member] | Settled Interest Rate Swap 2 [Member]            
Derivative [Line Items]            
Cash payment for settled interest-rate swap       73    
Not Designated as Hedging Instrument [Member] | Interest-Rate Swaps 1 and 3 [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   500    
Not Designated as Hedging Instrument [Member] | Interest-Rate Swaps 1 Through 5 [Member]            
Derivative [Line Items]            
Notional principal amount of interest-rate swap   1,600   1,600    
Not Designated as Hedging Instrument [Member] | Permitted to Offset Gross Derivative Asset with Financial Institutions [Member]            
Derivative [Line Items]            
Gross derivative liabilities   $ 52   $ 52   $ 347
Natural Gas [Member] | Commodity [Member] | Not Designated as Hedging Instrument [Member]            
Derivative [Line Items]            
Financial derivative transactions | Bcf       5   8