Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Derivative assets:
Total derivative assets — gross	$ 89.7	$ 101.1	$ 124.4
Gross amounts offset in the balance sheet	(27.2)	(29.0)	(34.3)
Cash collateral received	(2.1)	0.0	0.0
Total derivative assets — net	60.4	72.1	90.1
Derivative liabilities:
Total derivative liabilities — gross	(198.1)	(191.6)	(126.1)
Gross amounts offset in the balance sheet	27.2	29.0	34.3
Cash collateral pledged	11.0	29.3	10.0
Total derivative liabilities — net	(159.9)	(133.3)	(81.8)
Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Total derivative assets — gross	0.2	1.4	1.3
Derivative liabilities:
Total derivative liabilities — gross	(3.2)	(3.7)	(0.5)
Derivatives designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	16.6	17.4	2.3
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	13.9	17.4	2.3
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative assets:
Total derivative assets — gross	2.7	0.0	0.0
Derivative liabilities:
Total derivative liabilities — gross	(6.9)	(12.3)	(3.7)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	72.9	82.3	120.8
Derivative liabilities:
Total derivative liabilities — gross	(188.0)	(175.6)	(121.9)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	26.0	41.6	22.1
Derivative liabilities:
Total derivative liabilities — gross	(4.4)	(4.3)	(8.8)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	46.9	40.7	98.7
Derivative liabilities:
Total derivative liabilities — gross	$ (183.6)	$ (171.3)	$ (113.1)