Impact of the Tax Cuts and Jobs Act of 2017 (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Income Tax Examination [Line Items]
TCJA Income Tax Benefit		$ 161.9
Regulated operations recorded as a regulatory liability	$ 740.9		$ 744.9
TCJA additional liability due to tax rates used in rates	$ 19.3		$ 22.5
Minimum
Income Tax Examination [Line Items]
Return issuance period	13 years
Maximum
Income Tax Examination [Line Items]
Return issuance period	51 years