Derivative Instruments, Hedging Activities and Fair Value Measurements, Recurring Fair Value Measurements (Details) - USD ($) $ in Millions		3 Months Ended						9 Months Ended
		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Total gains (losses) included in:
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities								$ (42.1)	$ 7.7
Fair Value, Measurements, Recurring [Member]
Financial assets [Abstract]
Interest rate derivatives		$ 3.5						3.5		$ 3.2
Commodity derivatives		245.7						245.7		255.6
Total		249.2						249.2		258.8
Financial liabilities [Abstract]
Liquidity Option Agreement		273.1						273.1		245.1
Interest rate derivatives		16.3						16.3		3.7
Commodity derivatives		475.4						475.4		143.0
Total		764.8						764.8		391.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0						0.0		0.0
Commodity derivatives		82.2						82.2		109.5
Total		82.2						82.2		109.5
Financial liabilities [Abstract]
Liquidity Option Agreement		0.0						0.0		0.0
Interest rate derivatives		0.0						0.0		0.0
Commodity derivatives		202.8						202.8		31.3
Total		202.8						202.8		31.3
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets [Abstract]
Interest rate derivatives		3.5						3.5		3.2
Commodity derivatives		149.1						149.1		145.2
Total		152.6						152.6		148.4
Financial liabilities [Abstract]
Liquidity Option Agreement		0.0						0.0		0.0
Interest rate derivatives		16.3						16.3		3.7
Commodity derivatives		256.6						256.6		109.2
Total		272.9						272.9		112.9
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets [Abstract]
Interest rate derivatives		0.0						0.0		0.0
Commodity derivatives		14.4						14.4		0.9
Total		14.4						14.4		0.9
Financial liabilities [Abstract]
Liquidity Option Agreement		273.1						273.1		245.1
Interest rate derivatives		0.0						0.0		0.0
Commodity derivatives		16.0						16.0		2.5
Total		289.1						289.1		$ 247.6
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Financial liability balance, net, beginning of period		(263.7)	$ (242.3)	$ (246.7)	$ (232.8)	$ (221.6)	$ (219.3)	(246.7)	(219.3)
Total gains (losses) included in:
Transfers out of Level 3		0.0	0.0	0.1	1.1	1.5	0.1
Financial liability balance, net, end of period		(274.7)	(263.7)	(242.3)	(251.5)	(232.8)	(221.6)	(274.7)	(251.5)
Unrealized gain (loss) recognized as a component of net income related to financial assets and liabilities								$ 0.5	$ (1.1)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Revenue [Member]
Total gains (losses) included in:
Net income	[1]	0.2	0.0	0.7	(0.3)	(0.4)	(0.4)
Settlements		(0.2)	(0.1)	(0.1)	0.3	0.2	(0.5)
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Expense [Member]
Total gains (losses) included in:
Net income		(6.9)	(23.3)	2.2	(4.3)	(11.5)	0.0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other Comprehensive Income (Loss) [Member]
Total gains (losses) included in:
Other comprehensive income		$ (4.1)	$ 2.0	$ 1.5	$ (15.5)	$ (1.0)	$ (1.5)

[1] There were $0.5 million of unrealized gains included in these amounts for the nine months ended September 30, 2016. There were unrealized losses of $1.1 million included in these amounts for the nine months ended September 30, 2015.