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Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2 $ 2
Interest cost 29 27
Expected return on plan assets (40) (45)
Amortization of unrecognized net (gain) loss 11 11
Amortization of unrecognized prior service benefit 0  
Settlement charge 0 4
Net periodic (benefit) cost 2 (1)
Other Postretirement Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Interest cost 1 1
Expected return on plan assets (1) (1)
Amortization of unrecognized net (gain) loss 0  
Amortization of unrecognized prior service benefit 0 (1)
Settlement charge 0  
Net periodic (benefit) cost $ 0 $ (1)