Employee Retirement Plans - Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 9.8	$ 11.2	$ 11.9
Interest cost	27.3	26.3	25.0
Expected return on assets	(36.3)	(35.0)	(33.6)
Curtailment gain	(0.4)	(0.2)	(1.4)
Amortization of:
Prior service cost (benefit)	0.3	0.3	0.3
Actuarial loss (gain)	7.8	13.4	16.7
Net benefit cost (benefit)	8.5	16.0	18.9
Change in associated regulatory liabilities	0.0	0.0	0.0
Net benefit cost (benefit) after change in regulatory liabilities	8.5	16.0	18.9
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	0.2	0.5	1.0
Interest cost	0.7	0.8	0.8
Expected return on assets	(0.8)	(0.7)	(0.7)
Curtailment gain	0.0	0.0	0.0
Amortization of:
Prior service cost (benefit)	(0.5)	(6.3)	(0.6)
Actuarial loss (gain)	(0.1)	(0.1)	0.3
Net benefit cost (benefit)	(0.5)	(5.8)	0.8
Change in associated regulatory liabilities	(1.4)	(0.5)	(0.5)
Net benefit cost (benefit) after change in regulatory liabilities	$ (1.9)	$ (6.3)	$ 0.3