Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2020	Mar. 05, 2020	Dec. 31, 2019
Financial liabilities [Abstract]
Liquidity Option		$ 511.9
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Value before application of CME Rule 814	$ 1,663.9		$ 397.2
Impact of CME Rule 814	(1,448.9)		(269.4)
Total commodity derivatives	215.0		127.8
Total	215.0		127.8
Financial liabilities [Abstract]
Liquidity Option			509.6
Interest rate derivatives	250.5		13.5
Commodity derivatives:
Value before application of CME Rule 814	1,861.3		362.0
Impact of CME Rule 814	(1,664.6)		(245.8)
Total commodity derivatives	196.7		116.2
Total	447.2		639.3
Net value before application of CME Rule 814 to commodity hedging portfolio	(197.4)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814	999.1		53.4
Impact of CME Rule 814	(973.4)		(47.0)
Total commodity derivatives	25.7		6.4
Total	25.7		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	1,207.1		88.1
Impact of CME Rule 814	(1,181.6)		(81.9)
Total commodity derivatives	25.5		6.2
Total	25.5		6.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814	635.9		343.7
Impact of CME Rule 814	(451.5)		(222.4)
Total commodity derivatives	184.4		121.3
Total	184.4		121.3
Financial liabilities [Abstract]
Liquidity Option			0.0
Interest rate derivatives	250.5		13.5
Commodity derivatives:
Value before application of CME Rule 814	605.3		273.6
Impact of CME Rule 814	(447.9)		(163.9)
Total commodity derivatives	157.4		109.7
Total	407.9		123.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814	28.9		0.1
Impact of CME Rule 814	(24.0)		0.0
Total commodity derivatives	4.9		0.1
Total	4.9		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	48.9		0.3
Impact of CME Rule 814	(35.1)		0.0
Total commodity derivatives	13.8		0.3
Total	$ 13.8		$ 509.9