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Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2 $ 2 $ 6 $ 6
Interest cost 30 33 89 97
Expected return on plan assets (43) (45) (129) (133)
Amortization of unrecognized net loss 10 11 32 34
Settlement charge 7 1 10 3
Net periodic benefit cost 6 2 8 7
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost   1   1
Interest cost 1 1 2 2
Expected return on plan assets (1) (1) (3) (3)
Amortization of unrecognized prior service benefit (1) $ (1) (2) (3)
Net periodic benefit cost $ (1)   $ (3) $ (3)