Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financial assets [Abstract]
Interest rate derivatives		$ 12.4
Value before application of CME Rule 814	$ 1,877.8	1,570.1
Impact of CME Rule 814	(1,552.9)	(1,340.9)
Total commodity derivatives	324.9	229.2
Total	324.9	241.6
Financial liabilities [Abstract]
Interest rate derivatives		120.1
Commodity derivatives:
Value before application of CME Rule 814	2,218.5	2,138.9
Impact of CME Rule 814	(1,969.0)	(1,892.6)
Total commodity derivatives	249.5	246.3
Total	249.5	366.4
Net value before application of CME Rule 814 to commodity hedging portfolio	(340.7)
Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0
Value before application of CME Rule 814	719.7	678.6
Impact of CME Rule 814	(719.7)	(678.6)
Total commodity derivatives	0.0	0.0
Total	0.0	0.0
Financial liabilities [Abstract]
Interest rate derivatives		0.0
Commodity derivatives:
Value before application of CME Rule 814	1,270.7	1,065.6
Impact of CME Rule 814	(1,270.7)	(1,065.6)
Total commodity derivatives	0.0	0.0
Total	0.0	0.0
Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		12.4
Value before application of CME Rule 814	1,155.6	878.6
Impact of CME Rule 814	(831.9)	(650.4)
Total commodity derivatives	323.7	228.2
Total	323.7	240.6
Financial liabilities [Abstract]
Interest rate derivatives		120.1
Commodity derivatives:
Value before application of CME Rule 814	947.3	1,047.4
Impact of CME Rule 814	(698.2)	(807.3)
Total commodity derivatives	249.1	240.1
Total	249.1	360.2
Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0
Value before application of CME Rule 814	2.5	12.9
Impact of CME Rule 814	(1.3)	(11.9)
Total commodity derivatives	1.2	1.0
Total	1.2	1.0
Financial liabilities [Abstract]
Interest rate derivatives		0.0
Commodity derivatives:
Value before application of CME Rule 814	0.5	25.9
Impact of CME Rule 814	(0.1)	(19.7)
Total commodity derivatives	0.4	6.2
Total	$ 0.4	$ 6.2