Derivative Instruments and Hedging Activities - Schedule of Derivative Assets, Liabilities and the Effects of Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Derivative assets:
Total derivative assets – gross	$ 101.1	$ 232.5
Gross amounts offset in the balance sheet	(29.0)	(34.3)
Cash collateral received	0.0	(12.2)
Total derivative assets – net	72.1	186.0
Derivative liabilities:
Total derivative liabilities – gross	(191.6)	(58.8)
Gross amounts offset in the balance sheet	29.0	34.3
Cash collateral pledged	29.3	0.0
Total derivative liabilities – net	(133.3)	(24.5)
Commodity contract - subject to PGC and DS mechanisms
Derivative assets:
Total derivative assets – gross	1.4	3.0
Derivative liabilities:
Total derivative liabilities – gross	(3.7)	(0.1)
Derivatives designated as hedging instruments
Derivative assets:
Total derivative assets – gross	17.4	2.4
Derivative liabilities:
Total derivative liabilities – gross	(12.3)	(1.4)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets – gross	17.4	1.5
Derivative liabilities:
Total derivative liabilities – gross	0.0	(0.4)
Derivatives designated as hedging instruments | Cross-currency swaps
Derivative assets:
Total derivative assets – gross	0.0	0.9
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities – gross	(12.3)	(1.0)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets – gross	82.3	227.1
Derivative liabilities:
Total derivative liabilities – gross	(175.6)	(57.3)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets – gross	41.6	19.1
Derivative liabilities:
Total derivative liabilities – gross	(4.3)	(14.0)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets – gross	40.7	208.0
Derivative liabilities:
Total derivative liabilities – gross	$ (171.3)	$ (43.3)