Derivative Instruments and Hedging Activities, Balance Sheet Presentation Related to Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Fair Value Hedging [Member]
Derivatives, Fair Value [Line Items]
Fair Value Hedges, Net	$ 0		$ 0
Enogex [Member]
Derivatives, Fair Value [Line Items]
Assets	5.5	[1]	61.3	[2]
Liabilities	5.5	[1]	53.5	[2]
Enogex [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	0		5.2
Liabilities	0.5		0.3
Enogex [Member] | 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
Enogex [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Assets	5.5		56.1
Liabilities	5.0		53.2
Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Options [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets			2.4
Liabilities			2.8
Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.1		0.4
Liabilities	0		0
Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Fixed Swaps/Futures [Member] | Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Assets	5.0		49.9
Liabilities	4.7		49.9
Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	0.4		3.1
Liabilities	0.3		0.4
Enogex [Member] | Not Designated as Hedging Instrument [Member] | Natural gas [Member] | Physical [Member] | Noncurrent PRM [Member]
Derivatives, Fair Value [Line Items]
Assets			0.3
Liabilities			$ 0.1

[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, 2012.

[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, 2011.