Retirement Benefits - Narrative (Details) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($) Rate	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($) category Rate	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial (loss) gain on pension and postretirement benefit obligations	$ (35.8)		$ (36.6)	$ 0.4	$ 3.3
Cash contribution to purchase the annuity contract		$ 3.9	3.9
Non-cash pre-tax losses from termination of plan		$ 0.8	0.8
Contributions			0.3	1.3	2.9
Plan assets	446.9		446.9	480.2
Defined contribution savings plan - expense recognized			7.7	15.5	15.0
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial (loss) gain on pension and postretirement benefit obligations			33.0	(1.7)
Contributions			6.6	4.0
Projected benefit obligation	623.2		623.2	623.8	652.5
Plan assets	446.9		446.9	480.2	507.4
Under-funded status	176.3		176.3	143.6
Future amortization of prior service cost (credit)	(0.3)		$ (0.3)
Number of investment categories | category			2
Pension plans that are not fully funded - projected benefit obligation	590.5		$ 590.5	591.0
Pension plans that are not fully funded - accumulated benefit obligation	585.9		585.9	586.5
Pension plans that are not fully funded - fair value of plan assets	413.9		413.9	446.8
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Actuarial (loss) gain on pension and postretirement benefit obligations			0.6	(2.5)
Contributions			2.4	2.5
Projected benefit obligation	16.1		16.1	16.9	21.5
Plan assets	0.0		0.0	0.0	$ 0.0
Under-funded status	$ 16.1		$ 16.1	$ 16.9
Assumed health care cost trend rate, percent | Rate	6.50%		6.50%
Ultimate health care cost trend rate, percent | Rate	5.00%		5.00%