Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 05, 2020
Financial liabilities [Abstract]
Liquidity Option		$ 0.0	$ 509.6		$ 511.9
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		79.3	(27.2)	$ (17.8)
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives		12.4
Value before application of CME Rule 814		1,570.1	397.2
Impact of CME Rule 814		(1,340.9)	(269.4)
Total commodity derivatives		229.2	127.8
Total		241.6	127.8
Financial liabilities [Abstract]
Liquidity Option			509.6
Interest rate derivatives		120.1	13.5
Commodity derivatives:
Value before application of CME Rule 814		2,138.9	362.0
Impact of CME Rule 814		(1,892.6)	(245.8)
Total commodity derivatives		246.3	116.2
Total		366.4	639.3
Net value before application of CME Rule 814 to commodity hedging portfolio		(568.8)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0
Value before application of CME Rule 814		678.6	53.4
Impact of CME Rule 814		(678.6)	(47.0)
Total commodity derivatives		0.0	6.4
Total		0.0	6.4
Financial liabilities [Abstract]
Liquidity Option			0.0
Interest rate derivatives		0.0	0.0
Commodity derivatives:
Value before application of CME Rule 814		1,065.6	88.1
Impact of CME Rule 814		(1,065.6)	(81.9)
Total commodity derivatives		0.0	6.2
Total		0.0	6.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		12.4
Value before application of CME Rule 814		878.6	343.7
Impact of CME Rule 814		(650.4)	(222.4)
Total commodity derivatives		228.2	121.3
Total		240.6	121.3
Financial liabilities [Abstract]
Liquidity Option			0.0
Interest rate derivatives		120.1	13.5
Commodity derivatives:
Value before application of CME Rule 814		1,047.4	273.6
Impact of CME Rule 814		(807.3)	(163.9)
Total commodity derivatives		240.1	109.7
Total		360.2	123.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0
Value before application of CME Rule 814		12.9	0.1
Impact of CME Rule 814		(11.9)	0.0
Total commodity derivatives		1.0	0.1
Total		1.0	0.1
Financial liabilities [Abstract]
Liquidity Option			509.6
Interest rate derivatives		0.0	0.0
Commodity derivatives:
Value before application of CME Rule 814		25.9	0.3
Impact of CME Rule 814		(19.7)	0.0
Total commodity derivatives		6.2	0.3
Total		6.2	509.9
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(509.8)	(395.9)
Total gains (losses) included in:
Transfer out of Level 3 - Liquidity Option	[1]	511.9	0.0
Other transfers out of Level 3		17.1	7.6
Financial liability balance, net, end of period		(5.2)	(509.8)	$ (395.9)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		0.7	0.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income (loss)		(23.7)	(2.1)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[2]	(1.6)	3.7
Settlements	[2]	3.2	(3.5)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		$ (2.3)	$ (119.6)

[1] In March 2020, the Liquidity Option settled and was transferred out of Level 3.  See Note 17 for information regarding the Liquidity Option.

[2] There were $0.7 million and $0.2 million of unrealized gains included in these amounts for the years ended December 31, 2020 and 2019, respectively.