Derivative Instruments and Hedging Activities, Income Statement Presentation Related to Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months			$ 0.5
Enogex [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Additional Collateral, Aggregate Fair Value	0.1		0.1
Enogex [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	(0.5)	(2.3)	(0.2)	(9.3)
Amount Reclassifed from Accumulated OCI into Income	0	(10.7)	5.2	(20.5)
Amount Recognized in Income	0	0	0	0
Enogex [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	(3.1)	(3.1)	(5.1)	(5.4)
Enogex [Member] | NGL [Member] | Options [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI		(2.4)		(9.2)
Amount Reclassifed from Accumulated OCI into Income		(3.3)		(5.8)
Amount Recognized in Income		0		0
Enogex [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in OCI	(0.5)	0.1	(0.2)	(0.1)
Amount Reclassifed from Accumulated OCI into Income	0	(7.4)	5.2	(14.7)
Amount Recognized in Income	0	0	0	0
Enogex [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	0.6	(0.2)	1.0	(0.4)
Enogex [Member] | Sales [Member] | Natural gas [Member] | Physical [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount Recognized in Income	$ (3.7)	$ (2.9)	$ (6.1)	$ (5.0)