Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Derivative assets:
Total derivative assets — gross	$ 124.4	$ 232.5	$ 131.2
Gross amounts offset in the balance sheet	(34.3)	(34.3)	(32.5)
Cash collateral received	0.0	(12.2)	(12.0)
Total derivative assets — net	90.1	186.0	86.7
Derivative liabilities:
Total derivative liabilities — gross	(126.1)	(58.8)	(87.4)
Gross amounts offset in the balance sheet	34.3	34.3	32.5
Cash collateral pledged	10.0	0.0	0.0
Total derivative liabilities — net	(81.8)	(24.5)	(54.9)
Derivatives designated as hedging instruments
Derivative assets:
Total derivative assets — gross	2.3	2.4	1.2
Derivative liabilities:
Total derivative liabilities — gross	(3.7)	(1.4)	(8.6)
Derivatives designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	2.3	1.5	1.2
Derivative liabilities:
Total derivative liabilities — gross	0.0	(0.4)	(5.6)
Derivatives designated as hedging instruments | Cross-currency contracts
Derivative assets:
Total derivative assets — gross	0.0	0.9	0.0
Derivative liabilities:
Total derivative liabilities — gross	0.0	0.0	(0.9)
Derivatives designated as hedging instruments | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(3.7)	(1.0)	(2.1)
Derivatives subject to PGC and DS mechanisms | Commodity contracts
Derivative assets:
Total derivative assets — gross	1.3	3.0	0.4
Derivative liabilities:
Total derivative liabilities — gross	(0.5)	(0.1)	(2.3)
Derivatives not designated as hedging instruments
Derivative assets:
Total derivative assets — gross	120.8	227.1	129.6
Derivative liabilities:
Total derivative liabilities — gross	(121.9)	(57.3)	(76.5)
Derivatives not designated as hedging instruments | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	22.1	19.1	10.4
Derivative liabilities:
Total derivative liabilities — gross	(8.8)	(14.0)	(34.3)
Derivatives not designated as hedging instruments | Commodity contracts
Derivative assets:
Total derivative assets — gross	98.7	208.0	119.2
Derivative liabilities:
Total derivative liabilities — gross	$ (113.1)	$ (43.3)	$ (42.2)