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Derivative Instruments - Additional Information (Detail) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended
Aug. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Derivative [Line Items]        
Additional collateral if credit rating downgraded   $ 124    
Amount of collateral posted related to derivative instruments with credit-risk-related contingent features   49   $ 170
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)   $ (1,400)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]        
Derivative [Line Items]        
Cash payment for interest-rate swap agreements   101 $ 118  
Not Designated as Hedging Instrument [Member] | Interest-Rate Swap 3 [Member]        
Derivative [Line Items]        
Notional principal amount of interest-rate swap $ 200 100    
Not Designated as Hedging Instrument [Member] | Interest-Rate Swap 3 [Member] | Cash Payments To Amend Interest Rate Swap Agreements [Member]        
Derivative [Line Items]        
Cash payment for interest-rate swap agreements $ 10      
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