Derivative Instruments and Hedging Activities - Schedule of Derivative Assets, Liabilities and the Effects of Offsetting (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Derivative assets:
Total derivative assets – gross	$ 116.3	$ 72.7
Gross amounts offset in the balance sheet	(35.7)	(35.0)
Cash collateral received	(8.3)	(0.3)
Total derivative assets – net	72.3	37.4
Derivative liabilities:
Total derivative liabilities – gross	(82.5)	(105.4)
Gross amounts offset in the balance sheet	35.7	35.0
Total derivative liabilities – net	(46.8)	(70.4)
Derivatives designated as hedging instruments
Derivative liabilities:
Total derivative liabilities – gross	(10.7)	(6.8)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets – gross	3.2	17.8
Derivative liabilities:
Total derivative liabilities – gross	(5.5)	(2.4)
Derivatives designated as hedging instruments | Cross-currency swaps
Derivative liabilities:
Total derivative liabilities – gross	(2.9)	(0.5)
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities – gross	(2.3)	(3.9)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Total derivative assets – gross	1.7	4.5
Derivative liabilities:
Total derivative liabilities – gross	(1.5)	(0.5)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets – gross	111.4	50.4
Derivative liabilities:
Total derivative liabilities – gross	(70.3)	(98.1)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets – gross	102.4	50.4
Derivative liabilities:
Total derivative liabilities – gross	(37.6)	(98.1)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets – gross	9.0	0.0
Derivative liabilities:
Total derivative liabilities – gross	$ (32.7)	$ 0.0