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Pension and Other Postretirement Benefits - Schedule of Estimated Benefit Payments (Detail)
$ in Millions
Dec. 31, 2015
USD ($)
Pension Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2016 $ 143
2017 143
2018 148
2019 153
2020 158
2021 through 2025 836
Other Postretirement Benefits [Member]  
Defined Benefit Plan Disclosure [Line Items]  
2016 38
2017 38
2018 38
2019 38
2020 38
2021 through 2025 $ 189