Risk Management And Hedging Activities (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended							1 Months Ended	3 Months Ended					1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Southeast Texas [Member]	Mar. 31, 2012 Southeast Texas [Member]	Jan. 31, 2011 Southeast Texas [Member]	Sep. 30, 2012 Minimum [Member]	Dec. 31, 2011 Minimum [Member]	Jun. 30, 2012 Interest Rate Derivatives [Member]	Mar. 31, 2012 Interest Rate Derivatives [Member]	Sep. 30, 2012 Interest Rate Derivatives [Member]	Dec. 31, 2011 Interest Rate Derivatives [Member]	Sep. 30, 2012 Interest Rate Derivatives [Member] Minimum [Member]	Sep. 30, 2012 Interest Rate Derivatives [Member] Maximum [Member]	Mar. 08, 2012 Forward-Starting Interest Rate Swap [Member]
Derivative [Line Items]
Ownership interest percentage			66.67%	66.67%	33.33%
2011 Base Gas Hedge Fair Value	$ 2.8
Derivative maturity date	June, July and August 2013							Q2 2012
Deferred loss recognized	2.7													4.8
Derivative hedge amount	150.0					150.0	450.0			150.0	450.0			195.0
Interest rate swap agreements designated as cash flow hedging instruments											425.0
Mark-to-market interest rate swaps											25.0
Discontinuation of cash flow hedge									225.0
Derivative Instruments	47.3	41.2						300.0
Interest rate swap agreements reprice every thirty days										150.0
Effective fixed-rates lower range										2.94%
Effective fixed-rates higher range										2.99%
Derivative settlement amount														6.6
Derivative net liability position	27.4									150.0
Reduce in net liability due to offset of contract	24.2
Net liability position										7.0
Outstanding parental guarantees	25.0
Derivative, notional amount												$ 70.0	$ 80.0