Derivative Instruments and Hedging Activities, Income Statement Presentation Related to Derivative Instruments (Details) (Enogex [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009
Derivative Instruments, Gain (Loss) [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ 4.9
Additional Collateral, Aggregate Fair Value	2.1
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	(5.5)	[1]	(22.9)	(109.9)
NGL [Member] | Options [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	(8.4)	[1]	(9.7)	(56.4)
NGL [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI			1.7	(33.7)
Natural gas [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	2.9	[1]	(14.9)	(19.8)
Operating Revenues [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income	(40.2)		(28.4)	(12.2)
Amount Recognized in Income	0		0.2	(0.2)
Operating Revenues [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	(9.6)		(8.5)	(6.8)
Operating Revenues [Member] | NGL [Member] | Options [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income	(9.8)		1.2	1.7
Amount Recognized in Income	0		0	0
Operating Revenues [Member] | NGL [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income			(3.7)	12.6
Amount Recognized in Income			0	0
Operating Revenues [Member] | NGL [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income				(0.2)
Operating Revenues [Member] | Natural gas [Member] | Physical [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	(10.0)		(11.7)	(24.3)
Operating Revenues [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Reclassifed from Accumulated OCI into Income	(30.4)		(25.9)	(26.5)
Amount Recognized in Income	0		0.2	(0.2)
Operating Revenues [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	$ 0.4		$ 3.2	$ 17.7

[1] )The estimated net amount of gains or losses included in Accumulated Other Comprehensive Income at December 31, 2011 that is expected to be reclassified into income within the next 12 months is a loss of $4.9 million.