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Employer Contributions, Participant Contributions and Benefit Payments (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Pension Plans      
Defined Contribution Plan Disclosure [Line Items]      
Employer Contributions $ 4,215 $ 4,191 $ 3,700
Benefit Payments 4,410 4,246 3,764
Other Postretirement Benefit Plans, Defined Benefit      
Defined Contribution Plan Disclosure [Line Items]      
Employer Contributions 4,000 3,650 3,280
Participant Contributions 63 59 36
Benefit Payments 2,515 2,184 1,942
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Contribution Plan Disclosure [Line Items]      
Employer Contributions 40 53 53
Benefit Payments $ 40 $ 53 $ 53