Retirement Benefits - Assumed Health Care Cost Trend (Details) - USD ($) $ in Millions	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Retirement Benefits [Abstract]
One percentage point increase - Increase (decrease) in total of service and interest cost components	$ 0.1	$ 0.1	$ 0.1
One percentage point decrease - increase (decrease) in total of service and interest cost components	0.0	(0.1)	(0.1)
One percentage point increase - increase (decrease) in postretirement benefit obligation	1.2	1.5	2.1
One percentage point decrease - increase (decrease) in postretirement benefit obligation	$ (1.0)	$ (1.3)	$ (1.8)

No definition available.

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- Definition

Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

+ 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 (m)
-URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920

+ Details

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- Definition

Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate.

+ References

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- Definition

Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

+ References

+ Details

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- Definition

Amount of increase in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point increase in assumed health care cost trend rate.

+ References

+ Details

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