Employee Retirement Plans - Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits
Defined Benefit Plan Disclosure
Service cost	$ 11.9	$ 10.1	$ 10.0
Interest cost	25.0	26.8	25.5
Expected return on assets	(33.6)	(32.4)	(32.2)
Curtailment gain	(1.4)	(1.2)	(0.8)
Amortization of:
Prior service cost (benefit)	0.3	0.3	0.3
Actuarial loss	16.7	10.9	10.0
Net benefit cost	18.9	14.5	12.8
Change in associated regulatory liabilities	0.0	0.0	0.0
Net benefit cost after change in regulatory liabilities	18.9	14.5	12.8
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	1.0	0.7	0.7
Interest cost	0.8	0.9	0.8
Expected return on assets	(0.7)	(0.6)	(0.6)
Curtailment gain	0.0	0.0	0.0
Amortization of:
Prior service cost (benefit)	(0.6)	(0.6)	(0.5)
Actuarial loss	0.3	0.0	0.1
Net benefit cost	0.8	0.4	0.5
Change in associated regulatory liabilities	(0.5)	1.0	3.7
Net benefit cost after change in regulatory liabilities	$ 0.3	$ 1.4	$ 4.2