Derivative Instruments, Hedging Activities and Fair Value Measurements, Derivative Fair Value Amounts (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 250.5	$ 13.5
Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	215.0	127.8
Liability Derivatives	196.7	116.2
Derivatives designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	167.1	116.5
Liability Derivatives	428.9	120.6
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0	0.0
Liability Derivatives	250.5	13.5
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0	0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	0.0	0.0
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	200.6	6.7
Derivatives designated as hedging instruments [Member] | Interest rate derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	49.9	6.8
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	167.1	116.5
Liability Derivatives	178.4	107.1
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	165.3	116.5
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	1.8	0.0
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	168.2	107.1
Derivatives designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	10.2	0.0
Derivatives not designated as hedging instruments [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	47.9	11.3
Liability Derivatives	18.3	9.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	47.9	11.3
Liability Derivatives	18.3	9.1
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	44.4	10.7
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other assets [Member]
Derivatives, Fair Value [Line Items]
Asset Derivatives	3.5	0.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Current liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	16.6	8.6
Derivatives not designated as hedging instruments [Member] | Commodity derivatives [Member] | Other liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability Derivatives	$ 1.7	$ 0.5