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Defined Benefit Pension and Other Postretirement Plans - Additional Information (Detail)
$ in Millions
9 Months Ended
Sep. 30, 2018
USD ($)
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
Employer contributions $ 100
Other Pension Plan  
Defined Benefit Plan Disclosure [Line Items]  
Benefits paid 13
Other Benefits  
Defined Benefit Plan Disclosure [Line Items]  
Benefits paid $ 25