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Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1 $ 2 $ 3 $ 4
Interest cost 30 28 59 55
Expected return on plan assets (40) (45) (80) (90)
Amortization of unrecognized net loss 12 10 23 21
Settlement charge 2 3 2 7
Net periodic (benefit) cost 5 (2) 7 (3)
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost     1 1
Expected return on plan assets (1) (1) (2) (2)
Amortization of unrecognized prior service benefit       (1)
Net periodic (benefit) cost $ (1) $ (1) $ (1) $ (2)