Risk Management And Hedging Activities (Schedule Of Designated And Non-Designated Derivative Instruments In Statement Of Financial Position, Fair Value) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Unrealized gains on derivative instruments - current	$ (43.4)	$ (59.9)
Unrealized gains on derivative instruments - long-term	(13.6)	(32.8)
Unrealized gains on derivative instruments - current	47.3	41.2
Unrealized gains on derivative instruments - long-term	37.2	6.4
Derivative Assets Designated As Hedging Instruments [Member] | Commodity Derivatives [Member]
Derivative [Line Items]
Unrealized gains on derivative instruments - current	(2.8)
Unrealized gains on derivative instruments - long-term		(2.6)
Derivative liabilities, fair value, total	(2.8)	(2.6)
Derivative assets, fair value, total
Derivative Assets Designated As Hedging Instruments [Member] | Interest Rate Derivatives [Member]
Derivative [Line Items]
Unrealized gains on derivative instruments - current	(4.1)	(15.7)
Unrealized gains on derivative instruments - long-term	(2.9)	(5.0)
Derivative liabilities, fair value, total	(7.0)	(20.7)
Derivative assets, fair value, total
Derivative Assets Not Designated As Hedging Instruments [Member] | Commodity Derivatives [Member]
Derivative [Line Items]
Unrealized gains on derivative instruments - current	(36.5)	(43.8)
Unrealized gains on derivative instruments - long-term	(10.7)	(25.2)
Derivative liabilities, fair value, total	(47.2)	(69.0)
Unrealized gains on derivative instruments - current	47.3	41.2
Unrealized gains on derivative instruments - long-term	37.2	6.4
Derivative assets, fair value, total	84.5	47.6
Derivative Assets Not Designated As Hedging Instruments [Member] | Interest Rate Derivatives [Member]
Derivative [Line Items]
Unrealized gains on derivative instruments - current		(0.4)
Unrealized gains on derivative instruments - long-term
Derivative liabilities, fair value, total		$ (0.4)