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Postretirement Benefits - Estimated Future Payments for Pension Benefits and Other Postretirement Benefit Plans (Detail)
$ in Thousands
Dec. 31, 2017
USD ($)
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 20,375
2019 22,143
2020 23,840
2021 25,239
2022 27,090
2023-2027 160,075
Other Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 1,228
2019 1,278
2020 1,311
2021 1,344
2022 1,386
2023-2027 $ 7,515