Derivative Instruments and Hedging Activities, Quantitative Disclosures Related to Derivative Instruments (Details) (Enogex [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2011 barrels
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | NGL [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	300,000	[1]
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Natural gas [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	1,900,000	[1]
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Natural gas [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	1,300,000	[1]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	58,000,000	[2],[3],[4],[5]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	16,500,000	[2],[3],[4],[5]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	47,300,000	[2],[4]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	48,600,000	[2],[4]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Options [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	13,000,000	[2],[4]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Options [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	19,200,000	[2],[4]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Basis Swaps [Member] | Sales [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	8,100,000	[2],[4]
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Basis Swaps [Member] | Purchases [Member]
Derivative [Line Items]
Gross Notional Volume (Barrels or MMBtu)	7,700,000	[2],[4]
Fair Value Hedging [Member]
Derivative [Line Items]
Fair Value Hedges, Net	0
Natural gas [Member]
Derivative [Line Items]
Percent of Contract with Durations of One Year or Less	85.50%
Percent of Contracts With Durations of More Than One Year and Less Than Two Years	6.50%
Percent of Contracts Having a Duration of More Than Two Years	8.00%

[1] Natural gas in million British thermal units; NGLs in barrels.

[2] 85.5 percent of the natural gas contracts have durations of one year or less, 6.5 percent have durations of more than one year and less than two years and 8.0 percent have durations of more than two years.

[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] Natural gas in million British thermal units.

[5] 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.