Employee Retirement Plans - Net Periodic Pension Expense and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Pension Benefit
Defined Benefit Plan Disclosure
Service cost	$ 10.0	$ 9.4	$ 11.3
Interest cost	25.5	26.1	23.8
Expected return on assets	(32.2)	(29.7)	(27.8)
Curtailment gain	(0.8)	0.0	0.0
Amortization of:
Prior service cost (benefit)	(0.3)	(0.3)	(0.3)
Actuarial loss	10.0	7.7	15.1
Net benefit cost	12.8	13.8	22.7
Net benefit cost after change in regulatory liabilities	12.8	13.8	22.7
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	0.7	0.5	0.6
Interest cost	0.8	0.9	0.9
Expected return on assets	(0.6)	(0.6)	(0.5)
Curtailment gain	0.0	0.0	0.0
Amortization of:
Prior service cost (benefit)	0.5	0.5	0.3
Actuarial loss	0.1		0.4
Net benefit cost	0.5	0.3	1.1
Change in associated regulatory liabilities	3.7	3.7	3.3
Net benefit cost after change in regulatory liabilities	$ 4.2	$ 4.0	$ 4.4