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Net Periodic Benefit Cost (Components of net periodic pension cost and net periodic postretirement benefit cost) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Pension Plans, Defined Benefiit    
Defined Benefit Plan Disclosure [Line Items]    
Service costs $ 267 $ 269
Interest cost 3,699 3,884
Expected return on plan assets (3,973) (4,637)
Net amortization 569 589
Net periodic pension (benefit) cost 562 105
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Interest cost 20 29
Net amortization (40) (12)
Net periodic pension (benefit) cost $ (20) $ 17