Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016
Derivative assets:
Derivative asset, gross	$ 92.6	$ 72.7	$ 48.6
Gross amounts offset in the balance sheet	(31.8)	(35.0)	(26.4)
Cash collateral received	(0.2)	(0.3)	0.0
Total derivative assets — net	60.6	37.4	22.2
Derivative liabilities:
Derivative liability, gross	(42.7)	(105.4)	(117.1)
Gross amounts offset in the balance sheet	31.8	35.0	26.4
Cash collateral pledged	0.0	0.0	0.1
Total derivative liabilities — net	(10.9)	(70.4)	(90.6)
Derivatives designated as hedging instruments
Derivative assets:
Derivative asset, gross	17.2	17.8	11.6
Derivative liabilities:
Derivative liability, gross	(2.2)	(6.8)	(9.7)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Derivative asset, gross	14.2	17.8	11.6
Derivative liabilities:
Derivative liability, gross	0.0	(2.4)	(5.0)
Derivatives designated as hedging instruments | Cross-currency contracts
Derivative assets:
Derivative asset, gross	3.0	0.0	0.0
Derivative liabilities:
Derivative liability, gross	0.0	(0.5)	(1.3)
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Derivative liability, gross	(2.2)	(3.9)	(3.4)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative asset, gross	2.1	4.5	1.2
Derivative liabilities:
Derivative liability, gross	(0.2)	(0.5)	(3.5)
Derivatives not designated as hedging instruments
Derivative assets:
Derivative asset, gross	73.3	50.4	35.8
Derivative liabilities:
Derivative liability, gross	(40.3)	(98.1)	(103.9)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Derivative asset, gross	1.4	0.0	0.0
Derivative liabilities:
Derivative liability, gross	(1.5)	0.0	0.0
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Derivative asset, gross	71.9	50.4	35.8
Derivative liabilities:
Derivative liability, gross	$ (38.8)	$ (98.1)	$ (103.9)