Derivative Instruments and Hedging Activities, Balance Sheet Presentation Related to Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012 Enogex [Member]		Dec. 31, 2012 Enogex [Member] Designated as Hedging Instrument [Member]	Dec. 31, 2012 Enogex [Member] Designated as Hedging Instrument [Member] Natural gas [Member] Fixed Swaps/Futures [Member] Other Current Assets [Member]	Dec. 31, 2012 Enogex [Member] Not Designated as Hedging Instrument [Member]	Dec. 31, 2012 Enogex [Member] Not Designated as Hedging Instrument [Member] Natural gas [Member] Fixed Swaps/Futures [Member] Current PRM [Member]	Dec. 31, 2012 Enogex [Member] Not Designated as Hedging Instrument [Member] Natural gas [Member] Fixed Swaps/Futures [Member] Other Current Assets [Member]	Dec. 31, 2012 Enogex [Member] Not Designated as Hedging Instrument [Member] Natural gas [Member] Physical [Member] Current PRM [Member]
Derivatives, Fair Value [Line Items]
Assets	$ 0	$ 5.5	[1]	$ 0	$ 0	$ 5.5	$ 0.1	$ 5.0	$ 0.4
Liabilities		$ 5.5	[1]	$ 0.5	$ 0.5	$ 5.0	$ 0	$ 4.7	$ 0.3

[1] See Note 4 for a reconciliation of the Company's total derivatives fair value to the Company's Condensed Consolidated Balance Sheet at December 31, 2012.