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Employer Contributions, Participant Contributions and Benefit Payments (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Plans      
Defined Contribution Plan Disclosure [Line Items]      
Employer Contributions $ 4,100 $ 5,146 $ 4,215
Benefit Payments 5,574 4,900 4,410
Other Postretirement Benefit Plans, Defined Benefit      
Defined Contribution Plan Disclosure [Line Items]      
Employer Contributions 4,000 4,000 4,000
Participant Contributions 126 61 63
Benefit Payments 2,405 2,421 2,515
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Contribution Plan Disclosure [Line Items]      
Employer Contributions 34 34 40
Benefit Payments $ 34 $ 34 $ 40