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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, 2020 | Mar. 31, 2019 | Mar. 31, 2018
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 | $ 410,181 | $ 395,202 |
Service cost | 1,139 | 1,078 | $ 2,580
Interest cost | 14,759 | 15,526 | 16,488
Expected return on plan assets | (15,887) | (18,454) | (21,483)
Net amortization | 2,279 | 2,339 | 3,083
Net periodic pension cost (benefit) | 2,290 | 489 | $ 690
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets [Abstract] |  |  |
Accumulated benefit obligation | 401,918 | 389,509 |
Asset categories: | $ 254,508 | $ 264,211 |
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] |  |  |
Discount rate | 2.79% | 3.42% | 3.49%
Expected long-term rate of return on plan assets | 5.01% | 5.77% | 6.77%
Rate of compensation increase on active plans | 0.59% | 0.61% | 0.39%
Postretirement Benefit Plans |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Interest cost | $ 71 | $ 92 | $ 126
Net amortization | (205) | (156) | (5)
Net periodic pension cost (benefit) | $ (134) | $ (64) | $ 121
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] |  |  |
Discount rate | 3.17% | 3.63% |

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