Income Taxes (Significant Components Of Deferred Tax Liabilities And Assets) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019
Deferred Tax Liabilities [Abstract]
Property, Plant and Equipment	$ 874,607		$ 861,278
Pension and Other Post-Retirement Benefit Costs	54,066		55,795
Other	23,377		54,486
Total Deferred Tax Liabilities	952,050		971,559
Deferred Tax Assets [Abstract]
Tax Loss and Credit Carryforwards	(179,363)		(175,542)
Pension and Other Post-Retirement Benefit Costs	(95,599)		(87,280)
Other	(44,239)		(55,355)
Total Gross Deferred Tax Assets	(319,201)		(318,177)
Valuation Allowance	63,205	$ 56,800	0
Total Deferred Tax Assets	(255,996)		(318,177)
Total Net Deferred Income Taxes	$ 696,054		$ 653,382