Derivative Instruments and Hedging Activities, Balance Sheet Presentation Related to Derivative Instruments (Details) (Enogex [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Assets	$ 0.5	[1]	$ 5.5	[2]
Liabilities	0.6	[1]	5.5	[2]
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets			0
Liabilities			0.5
Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets			0
Liabilities			0.5
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	0.5		5.5
Liabilities	0.6		5.0
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets			0.1
Liabilities			0
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	0.1		5.0
Liabilities	0.2		4.7
Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.4		0.4
Liabilities	$ 0.4		$ 0.3

[1] See Note 5 for a reconciliation of the Company's total derivatives fair value to the Company's Condensed Consolidated Balance Sheet at March 31, 2013.

[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, 2012.