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Accumulated Other Comprehensive Loss (AOCL) - Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect $ (121,271) $ (123,057) $ (123,345)    
Income tax effect 8,090 9,164 10,695    
Accumulated other comprehensive loss (113,181) (113,893) (112,650) $ (117,682) $ (91,832)
Defined benefit plan liabilities [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect (124,085) (125,917) (121,910)    
Income tax effect 8,654 9,736 11,243    
Accumulated other comprehensive loss   (116,181) (110,667) (115,852) (77,921)
Gain (loss) on financial instruments [Member]          
Accumulated Other Comprehensive Income (Loss) [Line Items]          
Other comprehensive loss before income tax effect 2,814 2,860 (1,435)    
Income tax effect $ (564) (572) (548)    
Accumulated other comprehensive loss   $ 2,288 $ (1,983) $ (1,830) $ (1,679)