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Benefit Plans (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Components of net periodic benefit cost [Abstract]        
Contribution made to pension and postretirement plans     $ 0  
Pension Benefits [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost $ 6 $ 11 17 $ 22
Interest cost 273 322 599 651
Expected return on plan assets (819) (686) (1,506) (1,372)
Amortization of net loss (gain) (17) 10 0 33
Amortization of prior service cost 0 0 0 0
Net periodic benefit (557) (343) (890) (666)
Other Postretirement Benefits [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost 26 23 52 51
Interest cost 55 54 109 110
Expected return on plan assets (191) (190) (381) (381)
Amortization of net loss (gain) (89) (83) (178) (169)
Amortization of prior service cost 22 22 45 45
Net periodic benefit $ (177) $ (174) $ (353) $ (344)