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Derivative Instruments - Additional Information (Detail)
$ in Millions
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Jul. 31, 2017
USD ($)
Jun. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
Bcf
Sep. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
Bcf
Derivative [Line Items]          
Cash payment for interest-rate swap agreements     $ 160 $ 639  
Amount of collateral posted related to derivative instruments with credit-risk-related contingent features     159   $ 117
Not Designated as Hedging Instrument [Member]          
Derivative [Line Items]          
Aggregate fair value of derivative instruments with credit-risk-related contingent features for which a net liability position existed (net of collateral)     1,200   $ 1,400
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]          
Derivative [Line Items]          
Cash payment for interest-rate swap agreements     118 $ 275  
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps 3 and 7 and Portion Of Interest Rate Swap 2 [Member]          
Derivative [Line Items]          
Notional principal amount of interest-rate swap   $ 625      
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps 3 and 7 and Portion Of Interest Rate Swap 2 [Member] | Cash Payments To Amend Interest Rate Swap Agreements [Member]          
Derivative [Line Items]          
Cash payment for interest-rate swap agreements   $ 57      
Not Designated as Hedging Instrument [Member] | Interest-Rate Swap 1 [Member]          
Derivative [Line Items]          
Notional principal amount of interest-rate swap $ 125   550    
Not Designated as Hedging Instrument [Member] | Interest-Rate Swap 1 [Member] | Cash Payments To Amend Interest Rate Swap Agreements [Member]          
Derivative [Line Items]          
Cash payment for interest-rate swap agreements $ 15        
Not Designated as Hedging Instrument [Member] | Interest-Rate Swaps 1 Through 7 [Member]          
Derivative [Line Items]          
Notional principal amount of interest-rate swap     $ 1,600    
Natural Gas [Member] | Commodity [Member] | Not Designated as Hedging Instrument [Member]          
Derivative [Line Items]          
Financial derivative transactions | Bcf     15   2