Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2020	Mar. 05, 2020	Dec. 31, 2019
Financial liabilities [Abstract]
Liquidity Option		$ 511.9
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives	$ 5.7
Value before application of CME Rule 814	949.2		$ 397.2
Impact of CME Rule 814	(809.8)		(269.4)
Total commodity derivatives	139.4		127.8
Total	145.1		127.8
Financial liabilities [Abstract]
Liquidity Option			509.6
Interest rate derivatives	193.6		13.5
Commodity derivatives:
Value before application of CME Rule 814	1,306.0		362.0
Impact of CME Rule 814	(1,115.8)		(245.8)
Total commodity derivatives	190.2		116.2
Total	383.8		639.3
Net value before application of CME Rule 814 to commodity hedging portfolio	(356.8)
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives	0.0
Value before application of CME Rule 814	442.4		53.4
Impact of CME Rule 814	(417.8)		(47.0)
Total commodity derivatives	24.6		6.4
Total	24.6		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	637.9		88.1
Impact of CME Rule 814	(613.6)		(81.9)
Total commodity derivatives	24.3		6.2
Total	24.3		6.2
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives	5.7
Value before application of CME Rule 814	454.1		343.7
Impact of CME Rule 814	(352.3)		(222.4)
Total commodity derivatives	101.8		121.3
Total	107.5		121.3
Financial liabilities [Abstract]
Liquidity Option			0.0
Interest rate derivatives	193.6		13.5
Commodity derivatives:
Value before application of CME Rule 814	567.9		273.6
Impact of CME Rule 814	(433.5)		(163.9)
Total commodity derivatives	134.4		109.7
Total	328.0		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	52.7		0.1
Impact of CME Rule 814	(39.7)		0.0
Total commodity derivatives	13.0		0.1
Total	13.0		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	100.2		0.3
Impact of CME Rule 814	(68.7)		0.0
Total commodity derivatives	31.5		0.3
Total	$ 31.5		$ 509.9