Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 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	$ 1,716.9		$ 397.2
Impact of CME Rule 814	(1,346.0)		(269.4)
Total commodity derivatives	370.9		127.8
Total	370.9		127.8
Financial liabilities [Abstract]
Liquidity Option			509.6
Interest rate derivatives	258.3		13.5
Commodity derivatives:
Value before application of CME Rule 814	1,449.3		362.0
Impact of CME Rule 814	(1,177.1)		(245.8)
Total commodity derivatives	272.2		116.2
Total	530.5		639.3
Net value before application of CME Rule 814 to commodity hedging portfolio	267.6
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814	764.5		53.4
Impact of CME Rule 814	(723.9)		(47.0)
Total commodity derivatives	40.6		6.4
Total	40.6		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	531.1		88.1
Impact of CME Rule 814	(490.9)		(81.9)
Total commodity derivatives	40.2		6.2
Total	40.2		6.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814	938.1		343.7
Impact of CME Rule 814	(609.7)		(222.4)
Total commodity derivatives	328.4		121.3
Total	328.4		121.3
Financial liabilities [Abstract]
Liquidity Option			0.0
Interest rate derivatives	258.3		13.5
Commodity derivatives:
Value before application of CME Rule 814	912.9		273.6
Impact of CME Rule 814	(685.5)		(163.9)
Total commodity derivatives	227.4		109.7
Total	485.7		123.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814	14.3		0.1
Impact of CME Rule 814	(12.4)		0.0
Total commodity derivatives	1.9		0.1
Total	1.9		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	5.3		0.3
Impact of CME Rule 814	(0.7)		0.0
Total commodity derivatives	4.6		0.3
Total	$ 4.6		$ 509.9