Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Derivative assets:
Derivative asset, gross	$ 154.0	$ 72.7	$ 58.4
Gross amounts offset in the balance sheet	(35.7)	(35.0)	(15.6)
Cash collateral received	(7.1)	(0.3)	0.0
Total derivative assets — net	111.2	37.4	42.8
Derivative liabilities:
Derivative liability, gross	(68.5)	(105.4)	(177.8)
Gross amounts offset in the balance sheet	35.7	35.0	15.6
Cash collateral pledged	0.0	0.0	5.5
Total derivative liabilities — net	(32.8)	(70.4)	(156.7)
Derivatives designated as hedging instruments
Derivative assets:
Derivative asset, gross	28.1	17.8	27.9
Derivative liabilities:
Derivative liability, gross	(2.8)	(6.8)	(9.8)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Derivative asset, gross	24.6	17.8	25.4
Derivative liabilities:
Derivative liability, gross	0.0	(2.4)	0.0
Derivatives designated as hedging instruments | Cross-currency contracts
Derivative assets:
Derivative asset, gross	3.5	0.0	1.9
Derivative liabilities:
Derivative liability, gross	0.0	(0.5)	0.0
Derivatives designated as hedging instruments | Interest rate contracts
Derivative assets:
Derivative asset, gross	0.0	0.0	0.6
Derivative liabilities:
Derivative liability, gross	(2.8)	(3.9)	(9.8)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Derivative asset, gross	6.9	4.5	0.2
Derivative liabilities:
Derivative liability, gross	(0.3)	(0.5)	(6.3)
Derivatives not designated as hedging instruments
Derivative assets:
Derivative asset, gross	119.0	50.4	30.3
Derivative liabilities:
Derivative liability, gross	(65.4)	(98.1)	(161.7)
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	(0.2)	0.0	0.0
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Derivative asset, gross	117.6	50.4	30.3
Derivative liabilities:
Derivative liability, gross	$ (65.2)	$ (98.1)	$ (161.7)