Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		$ (12.5)		$ (322.1)		$ 83.8	$ (459.0)
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Value before application of CME Rule 814		408.4				408.4		$ 456.9
Impact of CME Rule 814		(206.1)				(206.1)		(295.0)
Total commodity derivatives		202.3				202.3		161.9
Total		202.3				202.3		161.9
Financial liabilities [Abstract]
Liquidity option agreement		474.4				474.4		390.0
Interest rate derivatives		5.2				5.2
Commodity derivatives:
Value before application of CME Rule 814		353.6				353.6		398.1
Impact of CME Rule 814		(216.6)				(216.6)		(235.7)
Total commodity derivatives		137.0				137.0		162.4
Total		616.6				616.6		552.4
Net value before application of CME Rule 814 to commodity hedging portfolio		54.8				54.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814		62.7				62.7		172.3
Impact of CME Rule 814		(46.7)				(46.7)		(134.8)
Total commodity derivatives		16.0				16.0		37.5
Total		16.0				16.0		37.5
Financial liabilities [Abstract]
Liquidity option agreement		0.0				0.0		0.0
Interest rate derivatives		0.0				0.0
Commodity derivatives:
Value before application of CME Rule 814		77.6				77.6		85.5
Impact of CME Rule 814		(62.0)				(62.0)		(48.6)
Total commodity derivatives		15.6				15.6		36.9
Total		15.6				15.6		36.9
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814		339.4				339.4		282.4
Impact of CME Rule 814		(154.2)				(154.2)		(159.3)
Total commodity derivatives		185.2				185.2		123.1
Total		185.2				185.2		123.1
Financial liabilities [Abstract]
Liquidity option agreement		0.0				0.0		0.0
Interest rate derivatives		5.2				5.2
Commodity derivatives:
Value before application of CME Rule 814		253.4				253.4		291.2
Impact of CME Rule 814		(138.3)				(138.3)		(172.9)
Total commodity derivatives		115.1				115.1		118.3
Total		120.3				120.3		118.3
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Value before application of CME Rule 814		6.3				6.3		2.2
Impact of CME Rule 814		(5.2)				(5.2)		(0.9)
Total commodity derivatives		1.1				1.1		1.3
Total		1.1				1.1		1.3
Financial liabilities [Abstract]
Liquidity option agreement		474.4				474.4		390.0
Interest rate derivatives		0.0				0.0
Commodity derivatives:
Value before application of CME Rule 814		22.6				22.6		21.4
Impact of CME Rule 814		(16.3)				(16.3)		(14.2)
Total commodity derivatives		6.3				6.3		7.2
Total		480.7				480.7		$ 397.2
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(446.9)	$ (395.9)	(341.9)	$ (332.7)	(395.9)	(332.7)
Total gains (losses) included in:
Transfers out of Level 3		0.0	(0.2)	0.0	0.0
Financial liability balance, net, end of period		(479.6)	(446.9)	(349.0)	(341.9)	(479.6)	(349.0)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(3.2)		1.8		$ (0.2)	$ 0.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[1]	(0.1)	3.1	1.3	(0.5)
Settlements	[1]	(3.1)	(0.1)	0.5	(1.2)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		(26.6)	(57.8)	(8.9)	(7.5)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income		$ (2.9)	$ 4.0	$ 0.0	$ 0.0

[1] There were unrealized losses of $3.2 million and $0.2 million included in these amounts for the three and six months ended June 30, 2019, respectively.  There were unrealized losses of $1.8 million and $0.1 million included in these amounts for the three and six months ended June 30, 2018, respectively.