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Derivative Instruments - Additional Information (Detail) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Mar. 31, 2019
Dec. 31, 2018
Derivative [Line Items]        
Amount of collateral posted related to derivative instruments with credit-risk-related contingent features $ 264     $ 66
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) 1,400     1,100
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]        
Derivative [Line Items]        
Cash payment for interest-rate swap agreements 30 $ 48    
Interest Rate Swaps 1 Through 5 [Member] | Not Designated as Hedging Instrument [Member]        
Derivative [Line Items]        
Notional principal amount of interest-rate swap $ 1,600      
Interest Rate Swaps 6 Through 8 [Member] | Not Designated as Hedging Instrument [Member] | WES [Member]        
Derivative [Line Items]        
Notional principal amount of interest-rate swap     $ 375 $ 750