Derivative Instruments and Hedging Activities - Derivative Instruments Reconciliation (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Offsetting Derivative Assets [Abstract]
Fair value of derivative assets presented above	$ 266.5	$ 49.2
Fair value of cash margin receivable offset with derivatives	(135.4)	(9.0)
Netting of assets and liabilities with the same counterparty	(73.0)	(23.0)
Derivative asset after the effects of netting	58.1	17.2
Current Assets – Other	57.0	15.8
Deferred Charges and Other Assets – Other	1.1	1.4
Offsetting Derivative Liabilities [Abstract]
Fair value of derivative liabilities presented above	164.7	99.5
Netting of assets and liabilities with the same counterparty	(73.0)	(23.0)
Derivative liability after the effects of netting	91.7	76.5
Current Liabilities – Other	77.5	71.0
Deferred Credits and Other Liabilities – Other	$ 14.2	$ 5.5