Derivative Instruments and Hedging Activities, Income Statement Presentation Related to Derivative Instruments (Details) (Enogex [Member], USD $) In Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months				$ 8.4
Additional Collateral, Aggregate Fair Value	6.1			6.1
Natural Gas Liquids [Member] | Fixed Swaps/Futures [Member] | Operating Revenues [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income			(0.3)			(2.2)
Amount Recognized in Income			0			0
Natural Gas Liquids [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI			(1.2)			2.1
Natural Gas Liquids [Member] | Options [Member] | Operating Revenues [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income	(2.6)		1.5	(8.3)		2.0
Amount Recognized in Income	0		0	0		0
Natural Gas Liquids [Member] | Options [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	0.2	[1]	(12.2)	(9.0)	[1]	(1.2)
Natural gas [Member] | Physical [Member] | Operating Revenues [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	(2.2)		(2.3)	(7.1)		(6.4)
Natural gas [Member] | Fixed Swaps/Futures [Member] | Operating Revenues [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income	(7.5)		(6.7)	(22.2)		(18.7)
Amount Recognized in Income	0		0	0		0.1
Natural gas [Member] | Fixed Swaps/Futures [Member] | Operating Revenues [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	0.2		0.6	(0.2)		0.8
Natural gas [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	0.2	[1]	(5.5)	0	[1]	(15.4)
Operating Revenues [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income	(10.1)		(5.5)	(30.5)		(18.9)
Amount Recognized in Income	0		0	0		0.1
Operating Revenues [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	(2.0)		(1.7)	(7.3)		(5.6)
Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	$ 0.4	[1]	$ (18.9)	$ (9.0)	[1]	$ (14.5)

[1] The estimated net amount of gains or losses included in Accumulated Other Comprehensive Income at September 30, 2011 that is expected to be reclassified into income within the next 12 months is a loss of $8.4 million.