Defined Benefit Pension and Other Postretirement Plans - Components of Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 2.8	$ 2.6	$ 5.6	$ 5.1
Interest cost	5.8	6.8	11.6	13.6
Expected return on assets	(9.4)	(9.1)	(18.8)	(18.1)
Curtailment gain	(1.1)	0.0	(1.1)	0.0
Amortization of:
Prior service cost (benefit)	0.0		0.1	0.1
Actuarial loss	3.8	2.0	7.5	3.9
Net cost (benefit)	1.9	2.3	4.9	4.6
Change in associated regulatory liabilities	0.0	0.0	0.0	0.0
Net cost (benefit) after change in regulatory liabilities	1.9	2.3	4.9	4.6
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	0.0	0.1	0.1	0.1
Interest cost	0.1	0.2	0.3	0.4
Expected return on assets	(0.2)	(0.2)	(0.4)	(0.4)
Curtailment gain	0.0	0.0	0.0	0.0
Amortization of:
Prior service cost (benefit)	(0.1)	(0.2)	(0.2)	(0.3)
Actuarial loss	0.1	0.0	0.1	0.0
Net cost (benefit)	(0.1)	(0.1)	(0.1)	(0.2)
Change in associated regulatory liabilities	(0.4)	(0.4)	(0.7)	(0.7)
Net cost (benefit) after change in regulatory liabilities	$ (0.5)	$ (0.5)	$ (0.8)	$ (0.9)