Derivative Instruments and Hedging Activities, Balance Sheet Presentation Related to Derivative Instruments (Details) (Enogex [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Assets	$ 18.1	[1]	$ 61.3	[2]
Liabilities	18.9	[1]	53.5	[2]
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	0		5.2
Liabilities	0.5		0.3
Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	0		5.2
Liabilities	0.5		0.3
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	18.1		56.1
Liabilities	18.4		53.2
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Options [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	0.5		2.4
Liabilities	0.5		2.8
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.2		0.4
Liabilities	0.1		0
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	16.5		49.9
Liabilities	17.4		49.9
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.8		3.1
Liabilities	0.4		0.4
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Noncurrent PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.1		0.3
Liabilities	$ 0		$ 0.1

[1] See Note 3 for a reconciliation of the Company's total derivatives fair value to the Company's Condensed Consolidated Balance Sheet at June 30, 2012.

[2] See Note 3 for a reconciliation of the Company's total derivatives fair value to the Company's Condensed Consolidated Balance Sheet at December 31, 2011.