Derivative Instruments and Hedging Activities, Balance Sheet Presentation Related to Derivative Instruments (Details) (Enogex [Member], USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Assets	$ 39.4	[1]	$ 36.6	[2]
Liabilities	40.7	[1]	50.9	[2]
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	3.4		13.9
Liabilities	8.2		29.1
Designated as Hedging Instrument [Member] | NGL [Member] | Options [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	1.5		13.3
Liabilities	0		0
Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0		0
Liabilities	8.0		28.8
Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	1.9		0.6
Liabilities	0.2		0.3
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	36.0		22.7
Liabilities	32.5		21.8
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	1.8		1.4
Liabilities	0.4		1.2
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Noncurrent PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.3		0.8
Liabilities	0.1		0
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.1		0
Liabilities	0.1		0.1
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	33.5		20.0
Liabilities	31.7		19.8
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Options [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	0.3		0.5
Liabilities	$ 0.2		$ 0.7

[1] See Note 5 for a reconciliation of the Company's total derivatives fair value to the Company's Condensed Consolidated Balance Sheet at September 30, 2011.

[2] See Note 5 for a reconciliation of the Company's total derivatives fair value to the Company's Condensed Consolidated Balance Sheet at December 31, 2010.