Retirement Benefits - Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2019	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Benefit cost (income) associated with special events:
Cash contribution to purchase the annuity contract	$ 3.9	$ 3.9
Non-cash pre-tax losses from termination of plan	$ 0.8	0.8
Pension Benefits
Pension and Other Postretirement Benefits:
Service cost		0.5	$ 0.5	$ 1.0
Interest cost		21.9	23.6	24.3
Expected return on plan assets		(22.5)	(24.8)	(26.7)
Benefit cost (income) associated with special events:
Curtailment		0.0	0.0	(0.3)
Settlement		0.8	0.6	0.0
Recognition of actuarial losses (gains)		35.9	0.7	(1.1)
Net periodic benefit cost (income)		36.6	0.6	(2.8)
Non-cash net curtailment gain				0.3
Postretirement Benefits
Pension and Other Postretirement Benefits:
Service cost		0.0	0.0	0.0
Interest cost		0.7	0.8	1.0
Amortization of:
Prior service cost		(0.3)	(1.5)	(1.9)
Benefit cost (income) associated with special events:
Recognition of actuarial losses (gains)		(0.1)	(1.7)	(1.9)
Net periodic benefit cost (income)		$ 0.3	$ (2.4)	$ (2.8)