Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities						$ (459.0)	$ 43.9
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives		$ 13.0				13.0		$ 0.1
Value before application of CME Rule 814		321.7				321.7		234.9
Impact of CME Rule 814 change		(130.1)				(130.1)		(73.2)
Total commodity derivatives		191.6				191.6		161.7
Total		204.6				204.6		161.8
Financial liabilities [Abstract]
Liquidity option agreement		350.3				350.3		333.9
Interest rate derivatives		0.0				0.0		1.7
Commodity derivatives:
Value before application of CME Rule 814		852.1				852.1		390.7
Impact of CME Rule 814 change		(392.7)				(392.7)		(213.8)
Total commodity derivatives		459.4				459.4		176.9
Total		809.7				809.7		512.5
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0				0.0		0.0
Value before application of CME Rule 814		92.9				92.9		47.1
Impact of CME Rule 814 change		(6.9)				(6.9)		(47.1)
Total commodity derivatives		86.0				86.0		0.0
Total		86.0				86.0		0.0
Financial liabilities [Abstract]
Liquidity option agreement		0.0				0.0		0.0
Interest rate derivatives		0.0				0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		119.5				119.5		118.4
Impact of CME Rule 814 change		(34.3)				(34.3)		(118.4)
Total commodity derivatives		85.2				85.2		0.0
Total		85.2				85.2		0.0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		13.0				13.0		0.1
Value before application of CME Rule 814		224.0				224.0		184.9
Impact of CME Rule 814 change		(123.2)				(123.2)		(26.1)
Total commodity derivatives		100.8				100.8		158.8
Total		113.8				113.8		158.9
Financial liabilities [Abstract]
Liquidity option agreement		0.0				0.0		0.0
Interest rate derivatives		0.0				0.0		1.7
Commodity derivatives:
Value before application of CME Rule 814		729.1				729.1		270.6
Impact of CME Rule 814 change		(358.4)				(358.4)		(95.4)
Total commodity derivatives		370.7				370.7		175.2
Total		370.7				370.7		176.9
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0				0.0		0.0
Value before application of CME Rule 814		4.8				4.8		2.9
Impact of CME Rule 814 change		0.0				0.0		0.0
Total commodity derivatives		4.8				4.8		2.9
Total		4.8				4.8		2.9
Financial liabilities [Abstract]
Liquidity option agreement		350.3				350.3		333.9
Interest rate derivatives		0.0				0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		3.5				3.5		1.7
Impact of CME Rule 814 change		0.0				0.0		0.0
Total commodity derivatives		3.5				3.5		1.7
Total		353.8				353.8		$ 335.6
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(341.9)	$ (332.7)	$ (274.4)	$ (268.2)	(332.7)	(268.2)
Total gains (losses) included in:
Transfers out of Level 3		0.0	0.0	0.0	0.0
Financial liability balance, net, end of period		(349.0)	(341.9)	(293.5)	(274.4)	(349.0)	(293.5)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		1.8		(0.6)		$ 0.1	$ (1.3)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[1]	1.3	(0.5)	0.1	0.7
Settlements	[1]	0.5	(1.2)	(0.7)	(1.4)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		(8.9)	(7.5)	(18.6)	(5.5)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income		$ 0.0	$ 0.0	$ 0.1	$ 0.0

[1] There were unrealized gains of $1.8 million and $0.1 million included in these amounts for the three and six months ended June 30, 2018, respectively. There were unrealized losses of $0.6 million and $1.3 million included in these amounts for the three and six months ended June 30, 2017, respectively.