Derivative Instruments and Hedging Activities, Quantitative Disclosures Related to Derivative Instruments (Details) (Enogex [Member], Natural gas [Member])	Dec. 31, 2012
Derivative [Line Items]
Percent of Contract with Durations of One Year or Less	95.10%
Percent of Contracts With Durations of More Than One Year and Less Than Two Years	2.90%
Percent of Contracts Having a Duration of More Than Two Years	2.00%
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	3,700,000	[1]
Fixed Swaps/Futures [Member] | Not Designated as Hedging Instrument [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	17,900,000	[1],[2]
Fixed Swaps/Futures [Member] | Not Designated as Hedging Instrument [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	16,200,000	[1],[2]
Basis Swaps [Member] | Not Designated as Hedging Instrument [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	6,700,000	[1],[2]
Basis Swaps [Member] | Not Designated as Hedging Instrument [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	7,300,000	[1],[2]
Physical [Member] | Not Designated as Hedging Instrument [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	30,100,000	[1],[2],[3],[4]
Physical [Member] | Not Designated as Hedging Instrument [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (MMBtu's)	7,000,000	[1],[2],[3],[4]

[1] Natural gas in MMBtu's.

[2] 95.1 percent of the natural gas contracts have durations of one year or less, 2.9 percent have durations of more than one year and less than two years and 2.0 percent have durations of more than two years.

[3] 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.

[4] 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.