Impact of the Tax Cuts and Jobs Act of 2017 (Details) - USD ($) $ in Millions	9 Months Ended
	Jun. 30, 2019	Sep. 30, 2018
Income Tax Examination [Line Items]
TCJA Income Tax Benefit	$ 165.5
Regulated operations recorded as a regulatory liability	731.8	$ 744.9
TCJA additional regulatory liability due to tax rates used in rates	$ 6.1	$ 22.5
Minimum
Income Tax Examination [Line Items]
Return issuance period	13 years
Maximum
Income Tax Examination [Line Items]
Return issuance period	51 years