Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Derivative assets:
Total derivative assets — gross	$ 131.2	$ 116.3	$ 154.0
Gross amounts offset in the balance sheet	(32.5)	(35.7)	(35.7)
Cash collateral received	(12.0)	(8.3)	(7.1)
Total derivative assets — net	86.7	72.3	111.2
Derivative liabilities:
Total derivative liabilities — gross	(87.4)	(82.5)	(68.5)
Gross amounts offset in the balance sheet	32.5	35.7	35.7
Total derivative liabilities — net	(54.9)	(46.8)	(32.8)
Derivatives designated as hedging instruments
Derivative assets:
Total derivative assets — gross	1.2	3.2	28.1
Derivative liabilities:
Total derivative liabilities — gross	(8.6)	(10.7)	(2.8)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	1.2	3.2	24.6
Derivative liabilities:
Total derivative liabilities — gross	(5.6)	(5.5)	0.0
Derivatives designated as hedging instruments | Cross-currency contracts
Derivative assets:
Total derivative assets — gross	0.0	0.0	3.5
Derivative liabilities:
Total derivative liabilities — gross	(0.9)	(2.9)	0.0
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(2.1)	(2.3)	(2.8)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Total derivative assets — gross	0.4	1.7	6.9
Derivative liabilities:
Total derivative liabilities — gross	(2.3)	(1.5)	(0.3)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets — gross	129.6	111.4	119.0
Derivative liabilities:
Total derivative liabilities — gross	(76.5)	(70.3)	(65.4)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	10.4	9.0	1.4
Derivative liabilities:
Total derivative liabilities — gross	(34.3)	(32.7)	(0.2)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets — gross	119.2	102.4	117.6
Derivative liabilities:
Total derivative liabilities — gross	$ (42.2)	$ (37.6)	$ (65.2)