Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
		Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities						$ 43.9	$ (68.3)
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives		$ 31.7				31.7		$ 36.5
Commodity derivatives:
Value before application of CME Rule 814		177.9				177.9
Impact of CME Rule 814 change		(137.0)				(137.0)
Total commodity derivatives		40.9				40.9		541.4
Total		72.6				72.6		577.9
Financial liabilities [Abstract]
Liquidity option agreement		293.7				293.7		269.6
Interest rate derivatives		1.6				1.6		1.1
Commodity derivatives:
Value before application of CME Rule 814		144.3				144.3
Impact of CME Rule 814 change		(95.4)				(95.4)
Total commodity derivatives		48.9				48.9		739.4
Total		344.2				344.2		1,010.1
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0				0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		29.5				29.5
Impact of CME Rule 814 change		(29.5)				(29.5)
Total commodity derivatives		0.0				0.0		84.5
Total		0.0				0.0		84.5
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		9.8				9.8
Impact of CME Rule 814 change		(9.8)				(9.8)
Total commodity derivatives		0.0				0.0		136.8
Total		0.0				0.0		136.8
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		31.7				31.7		36.5
Commodity derivatives:
Value before application of CME Rule 814		147.9				147.9
Impact of CME Rule 814 change		(107.5)				(107.5)
Total commodity derivatives		40.4				40.4		455.2
Total		72.1				72.1		491.7
Financial liabilities [Abstract]
Liquidity option agreement		0.0				0.0		0.0
Interest rate derivatives		1.6				1.6		1.1
Commodity derivatives:
Value before application of CME Rule 814		134.2				134.2
Impact of CME Rule 814 change		(85.6)				(85.6)
Total commodity derivatives		48.6				48.6		602.3
Total		50.2				50.2		603.4
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0				0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		0.5				0.5
Impact of CME Rule 814 change		0.0				0.0
Total commodity derivatives		0.5				0.5		1.7
Total		0.5				0.5		1.7
Financial liabilities [Abstract]
Liquidity option agreement		293.7				293.7		269.6
Interest rate derivatives		0.0				0.0		0.0
Commodity derivatives:
Value before application of CME Rule 814		0.3				0.3
Impact of CME Rule 814 change		0.0				0.0
Total commodity derivatives		0.3				0.3		0.3
Total		294.0				294.0		$ 269.9
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(274.4)	$ (268.2)	$ (242.3)	$ (246.7)	(268.2)	(246.7)
Total gains (losses) included in:
Transfers out of Level 3		0.0	0.0	0.0	0.1
Financial liability balance, net, end of period		(293.5)	(274.4)	(263.7)	(242.3)	(293.5)	(263.7)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities		(0.7)		(0.1)		$ (1.3)	$ 0.5
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[1]	0.1	0.7	0.0	0.7
Settlements		(0.7)	(1.4)	(0.1)	(0.1)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		(18.6)	(5.5)	(23.3)	2.2
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income		$ 0.1	$ 0.0	$ 2.0	$ 1.5

[1] There were unrealized losses of $0.7 million and $1.3 million included in these amounts for the three and six months ended June 30, 2017, respectively. There were unrealized losses of $0.1 million and unrealized gains of $0.5 million included in these amounts for the three and six months ended June 30, 2016, respectively.