Retirement Benefits - Pension and Other Postretirement Benefits (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($) facility
Pension Benefits
Pension and Other Postretirement Benefits:
Service cost		$ 0.5	$ 1.0	$ 1.8
Interest cost		23.6	24.3	25.6
Expected return on plan assets		(24.8)	(26.7)	(27.1)
Amortization of:
Prior service cost		0.0	0.0	0.1
Benefit cost (income) associated with special events:
Curtailment		0.0	(0.3)	(1.4)
Contractual termination benefits		0.0	0.0	2.2
Settlement	$ 0.6	0.6	0.0	0.0
Recognition of actuarial losses (gains)		0.7	(1.1)	0.0
Net periodic benefit cost (income)		0.6	(2.8)	1.2
Non-cash net curtailment gain			0.3	1.0
Contractual termination benefits		0.0	0.0	$ 2.2
Number of manufacturing facilities closed | facility				1
Pension Benefits | Facility Closing
Benefit cost (income) associated with special events:
Curtailment				$ 0.4
Postretirement Benefits
Pension and Other Postretirement Benefits:
Service cost		0.0	0.0	0.1
Interest cost		0.8	1.0	1.1
Amortization of:
Prior service cost		(1.5)	(1.9)	(2.0)
Benefit cost (income) associated with special events:
Curtailment		0.0	0.0	0.4
Recognition of actuarial losses (gains)		(1.7)	(1.9)	(1.6)
Net periodic benefit cost (income)		$ (2.4)	$ (2.8)	$ (2.0)