Derivative Instruments and Hedging Activities, Balance Sheet Presentation Related to Derivative Instruments (Details) (Enogex [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Derivatives, Fair Value [Line Items]
Assets	$ 61.3	[1]	$ 36.6	[2]
Liabilities	53.5	[1]	50.9	[2]
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	5.2		13.9
Liabilities	0.3		29.1
Designated as Hedging Instrument [Member] | NGL [Member] | Options [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets			13.3
Liabilities			0
Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets			0
Liabilities			28.8
Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	5.2		0.6
Liabilities	0.3		0.3
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	56.1		22.7
Liabilities	53.2		21.8
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Options [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	2.4		0.5
Liabilities	2.8		0.7
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.4		0
Liabilities	0		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	49.9		20.0
Liabilities	49.9		19.8
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	3.1		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

[1] See Note 6 for a reconciliation of the Company's total derivatives fair value to the Company's Consolidated Balance Sheet at December 31, 2011.

[2] See Note 6 for a reconciliation of the Company's total derivatives fair value to the Company's Consolidated Balance Sheet at December 31, 2010.