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Net Periodic Benefit Cost (Components of net periodic pension cost and net periodic postretirement benefit cost) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Mar. 31, 2019
Pension Plans      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Projected Benefit Obligation $ 401,870 $ 410,181  
Service cost 1,092 1,139 $ 1,078
Interest cost 11,527 14,759 15,526
Expected return on plan assets (12,787) (15,887) (18,454)
Net amortization 3,234 2,279 2,339
Net periodic pension cost (benefit) 22,104 2,290 $ 489
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract]      
Accumulated benefit obligation $ 396,673 $ 401,918  
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]      
Discount rate 2.62% 2.79% 3.42%
Expected long-term rate of return on plan assets 4.60% 5.01% 5.77%
Rate of compensation increase on active plans 2.76% 2.76% 2.76%
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets $ 283,280 $ 254,508  
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Weighted-Average Interest Crediting Rate 1.10% 2.25% 2.25%
Defined Benefit Plan, Pension Plan with Projected Benefit Obligation in Excess of Plan Assets, Plan Assets $ 283,280 $ 257,093  
Postretirement Benefit Plans      
Defined Benefit Plan Disclosure [Line Items]      
Interest cost 46 71 $ 92
Net amortization (263) (205) (156)
Net periodic pension cost (benefit) $ (217) $ (134) $ (64)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]      
Discount rate 2.52% 3.17%