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Accumulated Other Comprehensive Loss (AOCL) - Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect $ (99,380) $ (123,057)
Income tax effect 7,638 9,164
Accumulated other comprehensive loss (91,742) (113,893)
Defined benefit plan liabilities [Member]    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect (102,053) (125,917)
Income tax effect 8,173 9,736
Gain (loss) on financial instruments [Member]    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Other comprehensive loss before income tax effect 2,673 2,860
Income tax effect $ (535) $ (572)