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Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2 $ 2 $ 6 $ 10
Interest cost 33 31 97 95
Expected return on plan assets (45) (48) (133) (145)
Amortization of unrecognized net loss 11 9 34 32
Settlement charge 1 2 3 2
Net periodic benefit cost 2 (4) 7 (6)
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 1 1 1 1
Interest cost 1 1 2 2
Expected return on plan assets (1) (1) (3) (3)
Amortization of unrecognized prior service benefit $ (1) (3) (3) (8)
Amortization of unrecognized net loss       1
Net periodic benefit cost   $ (2) $ (3) $ (7)