Pension and Other Postretirement Benefits - Amounts Showing in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Prior service (cost) credit	$ 506	$ 1,194
Net actuarial losses	(45,291)	(45,607)
Impact of adoption of ASU 2018-02	(2,931)	(2,931)
Deferred taxes	10,562	10,595
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	(36,749)
Prior service credit (cost)	(688)
Net actuarial (losses) gains	316
Deferred taxes	(33)
Total change for 2020	(405)
Ending balance	(37,154)
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service (cost) credit	(358)	(402)
Net actuarial losses	(41,336)	(41,100)
Impact of adoption of ASU 2018-02	(2,931)	(2,931)
Deferred taxes	10,728	10,781
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	(33,652)
Prior service credit (cost)	44
Net actuarial (losses) gains	(236)
Deferred taxes	(53)
Total change for 2020	(245)
Ending balance	(33,897)
Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Prior service (cost) credit	864	1,596
Net actuarial losses	(3,955)	(4,507)
Impact of adoption of ASU 2018-02	0	0
Deferred taxes	(166)	$ (186)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Beginning balance	(3,097)
Prior service credit (cost)	(732)
Net actuarial (losses) gains	552
Deferred taxes	20
Total change for 2020	(160)
Ending balance	$ (3,257)