Retirement Benefits - Significant Assumptions (Details)	12 Months Ended
Mar. 31, 2020 Rate	Mar. 31, 2019 Rate	Mar. 31, 2018 Rate
Pension Benefits
Benefit Obligations:
Discount rate	3.10%	3.70%	3.70%
Rate of compensation increase	3.00%	2.90%	2.90%
Net Periodic Benefit Cost:
Discount rate	3.70%	3.70%	3.90%
Rate of compensation increase	2.90%	2.90%	3.00%
Expected return on plan assets	4.90%	5.10%	5.30%
Postretirement Benefits
Benefit Obligations:
Discount rate	3.30%	3.90%	4.00%
Net Periodic Benefit Cost:
Discount rate	3.90%	4.00%	4.00%

Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
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Weighted average rate increase of compensation, used to determine benefit obligation of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
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-Section 50
-Paragraph 1
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Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

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Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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- Definition

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

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-SubTopic 20
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

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