Derivative Instruments, Hedging Activities and Fair Value Measurements, Fair Value Measurements (Details) (USD $) In Millions	3 Months Ended												9 Months Ended
	Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Sep. 30, 2011	Sep. 30, 2010
Financial liabilities:
Unrealized gain (loss) recognized as a component of net income related to level 3 financial assets and liabilities	$ 0.7						$ 6.4						$ 2.5	$ 4.1
Nonfinancial assets and liabilities [Abstract]
Fair value adjustments of certain property, plant and equipment	5.2						0						5.2	1.5
Reconciliation of changes in the fair value of Level 3 financial assets and liabilities [Roll Forward]
Balance, beginning of period	2.1		0.4		(25.9)		19.8		(2.6)		5.7		(25.9)	5.7
Total gains (losses) included in:
Net income	0.8	[1]	1.9	[1]	(0.5)	[1]	18.2	[1]	16.2	[1]	(3.6)	[1]
Other comprehensive income (loss)	0		0		16.2		(31.4)		22.2		(8.3)
Settlements	(2.2)		(0.2)		0.8		(16.1)		(16.2)		3.6
Transfers out of Level 3			0	[2]	9.8	[2]			0.2		0
Balance, end of period	0.7		2.1		0.4		(9.5)		19.8		(2.6)		0.7	(9.5)
Fair Value, Measurements, Recurring [Member]
Financial assets:
Interest rate derivatives	92.3												92.3
Commodity derivatives	86.1												86.1
Foreign currency derivatives	0.2												0.2
Total	178.6												178.6
Financial liabilities:
Interest rate derivatives	287.8												287.8
Commodity derivatives	105.5												105.5
Total	393.3												393.3
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Financial assets:
Interest rate derivatives	0												0
Commodity derivatives	43.9												43.9
Foreign currency derivatives	0												0
Total	43.9												43.9
Financial liabilities:
Interest rate derivatives	0												0
Commodity derivatives	49.6												49.6
Total	49.6												49.6
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Financial assets:
Interest rate derivatives	92.3												92.3
Commodity derivatives	36.9												36.9
Foreign currency derivatives	0.2												0.2
Total	129.4												129.4
Financial liabilities:
Interest rate derivatives	287.8												287.8
Commodity derivatives	51.3												51.3
Total	339.1												339.1
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Financial assets:
Interest rate derivatives	0												0
Commodity derivatives	5.3												5.3
Foreign currency derivatives	0												0
Total	5.3												5.3
Financial liabilities:
Interest rate derivatives	0												0
Commodity derivatives	4.6												4.6
Total	4.6												4.6
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
Nonfinancial assets and liabilities [Abstract]
Fair value adjustments of certain property, plant and equipment													$ 5.2	$ 1.5

[1] There were $0.7 million and $2.5 million of unrealized gains included in these amounts for the three and nine months ended September 30, 2011, respectively. There were $6.4 million and $4.1 million of unrealized gains included in these amounts for the three and nine months ended September 30, 2010, respectively.

[2] Transfers out of Level 3 into Level 2 were primarily due to the change in observability of forward NGL prices as described above.