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Derivative Instruments - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Derivative [Line Items]      
Additional collateral if credit rating downgraded $ 60    
Amount of collateral posted related to derivative instruments with credit-risk-related contingent features 69   $ 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 46 $ 24  
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps 1 Through 6 [Member]      
Derivative [Line Items]      
Notional principal amount of interest-rate swap $ 1,600