Derivative Instruments Summary (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivatives, Fair Value [Line Items]
Asset	$ 82,130	$ 86,233
Liability	(152,277)	(215,126)
Collateral Netting	9,474
Net Asset (Liability)	(60,673)	(128,893)
Other Current Liabilities [Member] | Foreign Currency Contract [Member]
Derivatives, Fair Value [Line Items]
Liability	(30)
Net Asset (Liability)	(30)
Other Current Liabilities [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Liability	(3,827)	(16,253)
Net Asset (Liability)	(3,827)	(16,253)
Other Intangibles, Property And Investments - Net [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Asset	5,069
Net Asset (Liability)	5,069
Current Utility Energy Commodity Derivative Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Asset	1,857	1,618
Liability	(179)	(479)
Net Asset (Liability)	1,678	1,139
Non-Current Utility Energy Commodity Derivative Assets [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Asset	1,709	185
Liability	(636)
Net Asset (Liability)	1,073	185
Current Utility Energy Commodity Derivative Liabilities [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Asset	46,879	40,090
Liability	(89,805)	(110,914)
Collateral Netting	7,138
Net Asset (Liability)	(35,788)	(70,824)
Other Non-Current Liabilities And Deferred Credits [Member] | Interest Rate Contract [Member]
Derivatives, Fair Value [Line Items]
Liability		(2,642)
Net Asset (Liability)		(2,642)
Other Non-Current Liabilities And Deferred Credits [Member] | Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Asset	26,616	44,308
Liability	(57,800)	(84,838)
Collateral Netting	2,336
Net Asset (Liability)	(28,848)	(40,530)
Other Current Assets [Member] | Foreign Currency Contract [Member]
Derivatives, Fair Value [Line Items]
Asset		32
Net Asset (Liability)		$ 32