Derivative Instruments and Hedging Activities - Derivative Instruments Reconciliation (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Offsetting Derivative Assets [Abstract]
Fair value of derivative assets presented above	$ 24.2	$ 27.4
Fair value of cash margin receivable offset with derivatives	14.3	6.4
Netting of assets and liabilities with the same counterparty	(23.7)	(14.8)
Derivative asset after the effects of netting	14.8	19.0
Current Assets – Other	4.2	13.3
Deferred Charges and Other Assets – Other	10.6	5.7
Offsetting Derivative Liabilities [Abstract]
Fair value of derivative liabilities presented above	78.5	21.0
Netting of assets and liabilities with the same counterparty	(23.7)	(14.8)
Derivative liability after the effects of netting	54.8	6.2
Current Liabilities – Other	50.0	6.0
Deferred Credits and Other Liabilities – Other	$ 4.8	$ 0.2