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%