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Changes in Fair Value Measurements that Used Significant Unobservable Inputs (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Beginning of period $ 4,501  
End of period 5,248 $ 4,501
Significant Unobservable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Beginning of period 16 8
Purchases, sales and settlements   8
Actual return on plan assets still held at year end 2  
End of period 18 16
Common/Collective Trust Funds    
Defined Benefit Plan Disclosure [Line Items]    
Beginning of period 900  
End of period 1,174 900
Common/Collective Trust Funds | Significant Unobservable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Beginning of period 6 6
Actual return on plan assets still held at year end 2  
End of period 8 6
Other Holdings    
Defined Benefit Plan Disclosure [Line Items]    
Beginning of period 117  
End of period 142 117
Other Holdings | Significant Unobservable Inputs (Level 3)    
Defined Benefit Plan Disclosure [Line Items]    
Beginning of period 10 2
Purchases, sales and settlements   8
End of period $ 10 $ 10