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, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 2.8	$ 3.0	$ 5.6	$ 6.0
Interest cost	6.5	6.1	13.0	12.3
Expected return on assets	(8.6)	(8.3)	(17.2)	(16.6)
Amortization of:
Prior service cost (benefit)	0.1		0.2	0.1
Actuarial loss	3.3	4.2	6.6	8.3
Net benefit cost	4.1	5.0	8.2	10.1
Change in associated regulatory liabilities	0.0	0.0	0.0	0.0
Net benefit cost after change in regulatory liabilities	4.1	5.0	8.2	10.1
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	0.2	0.3	0.4	0.5
Interest cost	0.2	0.2	0.4	0.4
Expected return on assets	(0.2)	(0.1)	(0.4)	(0.3)
Amortization of:
Prior service cost (benefit)		(0.2)	(0.1)	(0.3)
Actuarial loss	0.0	0.0	0.0	0.1
Net benefit cost	0.2	0.2	0.3	0.4
Change in associated regulatory liabilities	(0.1)	(0.1)	(0.2)	(0.2)
Net benefit cost after change in regulatory liabilities	$ 0.1	$ 0.1	$ 0.1	$ 0.2