Derivative Instruments and Hedging Activities, Quantitative Disclosures Related to Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Enogex [Member] | Natural gas [Member]
Derivative [Line Items]
Percent of Contract with Durations of One Year or Less	94.20%
Percent of Contracts With Durations of More Than One Year and Less Than Two Years	4.20%
Percent of Contracts Having a Duration of More Than Two Years	1.60%
Fixed Swaps/Futures [Member] | Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	100,000	[1],[2]
Fixed Swaps/Futures [Member] | Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	100,000	[1],[2]
Basis Swaps [Member] | Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	11,600,000	[1],[2]
Basis Swaps [Member] | Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	5,200,000	[1],[2]
Fair Value Hedging [Member]
Derivative [Line Items]
Fair Value Hedges, Net	$ 0		$ 0
Physical [Member] | Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	71,300,000	[1],[2],[3],[4]
Physical [Member] | Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	7,000,000	[1],[2],[3],[4]

[1] 94.2 percent of the natural gas contracts have durations of one year or less, 4.2 percent have durations of more than one year and less than two years and 1.6 percent have durations of more than two years.

[2] Natural gas in MMBtu's.

[3] Natural gas physical sales volumes exceed natural gas physical purchase volumes due to the marketing of natural gas volumes purchased via Enogex's processing contracts, which are not derivative instruments and are excluded from the table above.

[4] Of the natural gas physical purchases and sales volumes not designated as hedges, the majority are priced based on a monthly or daily index and the fair value is subject to little or no market price risk.