Derivative Instruments and Hedging Activities - Derivative Instruments Reconciliation (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Offsetting Derivative Assets [Abstract]
Fair value of derivative assets presented above	$ 49.2	$ 24.2
Fair value of cash margin receivable offset with derivatives	(9.0)	14.3
Netting of assets and liabilities with the same counterparty	(23.0)	(23.7)
Derivative asset after the effects of netting	17.2	14.8
Current Assets – Other	15.8	4.2
Deferred Charges and Other Assets – Other	1.4	10.6
Offsetting Derivative Liabilities [Abstract]
Fair value of derivative liabilities presented above	99.5	78.5
Netting of assets and liabilities with the same counterparty	(23.0)	(23.7)
Derivative liability after the effects of netting	76.5	54.8
Current Liabilities – Other	71.0	50.0
Deferred Credits and Other Liabilities – Other	$ 5.5	$ 4.8