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Defined Benefit Postretirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Pension Benefits [Member]    
Components of net periodic cost (benefit) [Abstract]    
Service cost $ 560 $ 655
Interest cost 1,051 998
Expected return on plan assets (1,835) (2,150)
Net amortization 483 546
Total cost (benefit) 259 49
Other Benefits [Member]    
Components of net periodic cost (benefit) [Abstract]    
Service cost 4 4
Interest cost 94 91
Expected return on plan assets 0 0
Net amortization 29 15
Total cost (benefit) $ 127 $ 110