Background and Basis of Presentation -Schedule of Impact of Change in Accounting Principle on Condensed Consolidated Statement of Operations and Condensed Consolidated Statements of Comprehensive Loss (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Sep. 30, 2020	Sep. 30, 2019		Sep. 30, 2020	Sep. 30, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Non-service defined benefit income	$ (12,427)	$ (27,824)	[1]	$ (24,843)	$ (41,226)	[1]
Loss before income taxes	(32,657)	53,461	[1]	(307,590)	74,883	[1]
Income tax expense	832	11,227	[1]	1,685	15,725	[1]
Net (loss) income	$ (33,489)	$ 42,234	[1]	$ (309,275)	$ 59,158	[1]
Net Income per share - basic & diluted
Basic	$ (0.64)	$ 0.84	[1]	$ (5.95)	$ 1.18	[1]
Diluted	$ (0.64)	$ 0.84	[1]	$ (5.95)	$ 1.17	[1]
Defined benefit pension plans and other postretirement benefits:
Total defined benefit pension plans and other postretirement benefits, net of taxes	$ 4,265	$ (68,852)	[1]	$ 8,530	$ (66,279)	[1]
Total other comprehensive income	13,778	(76,157)	[1]	21,062	(76,586)	[1]
Comprehensive loss	(19,711)	(33,923)	[1]	(288,213)	(17,428)	[1]
Previously Reported [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Non-service defined benefit income	(13,955)	(28,416)		(27,899)	(43,291)
Loss before income taxes	(31,129)	54,053		(304,534)	76,948
Income tax expense	832	11,352		1,685	16,159
Net (loss) income	$ (31,961)	$ 42,701		$ (306,219)	$ 60,789
Net Income per share - basic & diluted
Basic	$ (0.61)	$ 0.85		$ (5.90)	$ 1.22
Diluted	$ (0.61)	$ 0.85		$ (5.90)	$ 1.21
Defined benefit pension plans and other postretirement benefits:
Total defined benefit pension plans and other postretirement benefits, net of taxes	$ 2,737	$ (69,319)		$ 5,474	$ (67,910)
Total other comprehensive income	12,250	(76,624)		18,006	(78,217)
Comprehensive loss	(19,711)	(33,923)		(288,213)	(17,428)
Impact of Change [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Non-service defined benefit income	(1,528)	592		(3,056)	2,065
Loss before income taxes	1,528	(592)		3,056	(2,065)
Income tax expense		(125)			(434)
Net (loss) income	$ 1,528	$ (467)		$ 3,056	$ (1,631)
Net Income per share - basic & diluted
Basic	$ 0.03	$ (0.01)		$ 0.06	$ (0.03)
Diluted	$ 0.03	$ (0.01)		$ 0.06	$ (0.03)
Defined benefit pension plans and other postretirement benefits:
Total defined benefit pension plans and other postretirement benefits, net of taxes	$ (1,528)	$ 467		$ (3,056)	$ 1,631
Total other comprehensive income	$ (1,528)	$ 467		$ (3,056)	$ 1,631

[1] As adjusted; refer to Note 1.