Pension and Other Postretirement Benefits - Schedule of components of the net periodic benefit cost (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.4	$ 2.3	$ 4.9	$ 4.6
Interest cost	11.7	13.2	23.4	26.4
Expected return on plan assets	(15.5)	(16.2)	(31.1)	(32.4)
Amortization of net loss	3.9	2.6	7.8	5.2
Amortization of prior service cost (benefit)	0.5	0.5	1.0	1.0
Net periodic benefit costs	3.0	2.4	6.0	4.8
Other Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.7	0.6	1.4	1.2
Interest cost	2.3	2.5	4.6	5.1
Expected return on plan assets	(1.8)	(1.7)	(3.6)	(3.5)
Amortization of net loss	0.6	0.4	1.2	0.8
Amortization of prior service cost (benefit)	(1.0)	(1.3)	(2.0)	(2.6)
Net periodic benefit costs	$ 0.8	$ 0.5	$ 1.6	$ 1.0