Derivative Instruments and Hedging Activities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivative Line [Items]
Derivative financial instruments, fair value, net	$ 8.2
Cash flow commodity hedge deferred net gain to be reclassified within twelve months	10.8
Cash flow interest rate swaps deferred net loss to be reclassified within twelve months	5.7
Derivative Assets [Abstract]
Current assets	18.2		29.3
Long-term assets	3.2		5.1
Derivative Liabilities [Abstract]
Current liabilities	8.2		7.4
Long-term liabilities	5.0		4.8
Total derivative assets	21.4		34.4
Total derivative liabilities	13.2		12.2
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI - Effective Portion	(7.6)	15.5
Gain (Loss) Reclassified from OCI into Income - Effective Portion	4.9	0.2
Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from OCI into Income - Effective Portion	6.6	2.5
Interest Expense, Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Reclassified from OCI into Income - Effective Portion	(1.7)	(2.3)
Asset Position [Member] | Gross Presentation [Member]
Derivative Liabilities [Abstract]
Total current position	18.2		29.3
Total long-term position	3.2		5.1
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	19.5		28.2
Counterparties without offsetting position - assets	1.9		6.2
Counterparties without offsetting position - liabilities	0		0
Asset Position [Member] | Pro-forma Net Presentation [Member]
Derivative Liabilities [Abstract]
Total current position	11.1		21.9
Total long-term position	2.3		2.3
Total derivative assets	13.4		24.2
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	11.5		18.0
Counterparties without offsetting position - assets	1.9		6.2
Counterparties without offsetting position - liabilities	0		0
Liability Position [Member] | Gross Presentation [Member]
Derivative Liabilities [Abstract]
Total current position	8.2		7.4
Total long-term position	5.0		4.8
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	8.0		10.2
Counterparties without offsetting position - assets	0		0
Counterparties without offsetting position - liabilities	5.2		2.0
Liability Position [Member] | Pro-forma Net Presentation [Member]
Derivative Liabilities [Abstract]
Total current position	1.1		0
Total long-term position	4.1		2.0
Total derivative liabilities	5.2		2.0
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	0		0
Counterparties without offsetting position - assets	0		0
Counterparties without offsetting position - liabilities	5.2		2.0
Current Position [Member] | Asset Position [Member] | Gross Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	16.3		23.8
Counterparties without offsetting position - assets	1.9		5.5
Counterparties without offsetting position - liabilities	0		0
Current Position [Member] | Asset Position [Member] | Pro-forma Net Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	9.2		16.4
Counterparties without offsetting position - assets	1.9		5.5
Counterparties without offsetting position - liabilities	0		0
Current Position [Member] | Liability Position [Member] | Gross Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	7.1		7.4
Counterparties without offsetting position - assets	0		0
Counterparties without offsetting position - liabilities	1.1		0
Current Position [Member] | Liability Position [Member] | Pro-forma Net Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	0		0
Counterparties without offsetting position - assets	0		0
Counterparties without offsetting position - liabilities	1.1		0
Long-term Position [Member] | Asset Position [Member] | Gross Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	3.2		4.4
Counterparties without offsetting position - assets	0		0.7
Counterparties without offsetting position - liabilities	0		0
Long-term Position [Member] | Asset Position [Member] | Pro-forma Net Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	2.3		1.6
Counterparties without offsetting position - assets	0		0.7
Counterparties without offsetting position - liabilities	0		0
Long-term Position [Member] | Liability Position [Member] | Gross Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	0.9		2.8
Counterparties without offsetting position - assets	0		0
Counterparties without offsetting position - liabilities	4.1		2.0
Long-term Position [Member] | Liability Position [Member] | Pro-forma Net Presentation [Member]
Pro forma impact of reporting derivatives in the Consolidated Balance Sheet [Abstract]
Counterparties with offsetting position	0		0
Counterparties without offsetting position - assets	0		0
Counterparties without offsetting position - liabilities	4.1		2.0
Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) Recognized in OCI - Effective Portion	(7.6)	15.5
Unrealized gain (loss) on hedges, before tax	1.1		3.2
Unrealized gain (loss) on hedges, after tax	0.6		1.9
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (loss) on hedges, before tax	(1.0)		(1.2)
Unrealized gain (loss) on hedges, after tax	(0.6)		(0.7)
Designated as Hedging Instrument [Member]
Derivative Liabilities [Abstract]
Total derivative assets	21.3		34.3
Total derivative liabilities	13.1		12.0
Designated as Hedging Instrument [Member] | Commodity Contracts [Member]
Derivative Assets [Abstract]
Current assets	18.1		29.2
Long-term assets	3.2		5.1
Derivative Liabilities [Abstract]
Current liabilities	8.1		7.2
Long-term liabilities	5.0		4.8
Not Designated as Hedging Instrument [Member]
Derivative Liabilities [Abstract]
Total derivative assets	0.1		0.1
Total derivative liabilities	0.1		0.2
Not Designated as Hedging Instrument [Member] | Commodity Contracts [Member]
Derivative Assets [Abstract]
Current assets	0.1		0.1
Derivative Liabilities [Abstract]
Current liabilities	$ 0.1		$ 0.2
Instruments - Swaps [Member] | Year 2013 [Member] | Units - MMBtu/day [Member] | Natural Gas [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	31,108
Instruments - Swaps [Member] | Year 2013 [Member] | Unit - Bbl/D [Member] | NGL [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	5,650
Instruments - Swaps [Member] | Year 2013 [Member] | Unit - Bbl/D [Member] | Condensate [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	1,962
Instruments - Swaps [Member] | Year 2014 [Member] | Units - MMBtu/day [Member] | Natural Gas [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	25,500
Instruments - Swaps [Member] | Year 2014 [Member] | Unit - Bbl/D [Member] | NGL [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	1,000
Instruments - Swaps [Member] | Year 2014 [Member] | Unit - Bbl/D [Member] | Condensate [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	1,450
Instruments - Swaps [Member] | Year 2015 [Member] | Units - MMBtu/day [Member] | Natural Gas [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	12,001
Instruments - Swaps [Member] | Year 2015 [Member] | Unit - Bbl/D [Member] | NGL [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	0
Instruments - Swaps [Member] | Year 2015 [Member] | Unit - Bbl/D [Member] | Condensate [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	0
Instruments - Swaps [Member] | Year 2016 [Member] | Units - MMBtu/day [Member] | Natural Gas [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	5,000
Instruments - Swaps [Member] | Year 2016 [Member] | Unit - Bbl/D [Member] | NGL [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	0
Instruments - Swaps [Member] | Year 2016 [Member] | Unit - Bbl/D [Member] | Condensate [Member]
Derivative Line [Items]
Notional volumes of commodity hedges	0