Employee Benefit Plans - Schedule of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 10.1	$ 9.1	$ 10.5
Interest cost	17.1	18.7	17.9
Expected return on plan assets	(20.6)	(19.9)	(20.1)
Amortization of prior service cost (benefit)	1.6	1.7	1.8
Curtailment/settlement	0.1	1.2
Amortization of net actuarial loss (gain)	3.3	0.2	1.9
Expenses paid	4.0	2.5	1.9
Net periodic benefit cost	15.6	13.5	13.9
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial loss (gain)	29.4	63.3	(35.9)
Prior service cost (benefit)	9.8	0.2
Amortization of prior service benefit (cost)	(1.6)	(1.7)	(1.8)
Amortization of net actuarial (loss) gain	(3.3)	(0.2)	(1.9)
Total	$ 34.3	$ 61.6	$ (39.6)
Weighted-average assumptions
Discount rate	3.17%	4.18%	3.85%
Expected return on plan assets	5.49%	5.50%	5.93%
Postretirement Health and Other
Components of net periodic benefit cost
Service cost	$ 3.5	$ 3.1	$ 3.7
Interest cost	1.6	1.9	1.8
Amortization of prior service cost (benefit)	(0.9)	(1.5)	(0.9)
Amortization of net actuarial loss (gain)	(0.2)	(0.2)	0.1
Net periodic benefit cost	4.0	3.3	4.7
Other changes in plan assets and benefit obligations recognized in other comprehensive income
Net actuarial loss (gain)	5.5	1.4	(6.5)
Prior service cost (benefit)	(6.5)		(0.6)
Amortization of prior service benefit (cost)	0.9	1.5	0.9
Amortization of net actuarial (loss) gain	0.2	0.2	(0.1)
Total	$ 0.1	$ 3.1	$ (6.3)
Weighted-average assumptions
Discount rate	3.10%	4.20%	3.71%