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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 9 Months Ended
Dec. 31, 2020
Jun. 30, 2020
Dec. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]          
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement $ 113 $ (2,722)   $ (18,933)  
Pension Plans, Defined Benefiit          
Defined Benefit Plan Disclosure [Line Items]          
Service costs 294   $ 269 847 $ 784
Interest cost 2,792   3,701 8,823 11,077
Expected return on plan assets (3,045)   (3,971) (9,752) (11,917)
Net amortization 779   573 2,511 1,714
Net periodic pension (benefit) cost 707   572 21,362 1,658
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement     0   0
Other Postretirement Benefit Plans, Defined Benefit          
Defined Benefit Plan Disclosure [Line Items]          
Interest cost 7   14 35 53
Net amortization (94)   (74) (198) (154)
Net periodic pension (benefit) cost $ (87)   $ (60) $ (163) $ (101)