Fair Value (Schedule Of Carrying Amounts And Fair Values Of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 233,716	$ 209,620	$ 12,992	$ 7,483
Energy commodity derivatives deposits (current assets)	8,239	26,917
Energy commodity derivatives deposits (current liabilities)	(17,196)
Long-term receivables	3,097	2,534
Long-term receivables	3,065	2,510
Energy commodity derivatives contracts (current assets)	13,878	4,914
Forward-starting interest rate swap agreements (noncurrent)	1,008
Energy commodity derivatives contracts (noncurrent liabilities)	(2,008)	(6,457)
Debt	(2,148,432)	(2,151,775)
Debt	(2,426,955)	(2,389,700)
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	233,716	209,620
Energy commodity derivatives deposits (current assets)	8,239	26,917
Energy commodity derivatives deposits (current liabilities)	(17,196)
Long-term receivables	3,097	2,534
Energy commodity derivatives contracts (current assets)	13,878	4,914
Forward-starting interest rate swap agreements (noncurrent)	1,008
Energy commodity derivatives contracts (noncurrent liabilities)	(2,008)	(6,457)
Debt	(2,148,432)	(2,151,775)
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	233,716	209,620
Energy commodity derivatives deposits (current assets)	8,239	26,917
Energy commodity derivatives deposits (current liabilities)	(17,196)
Long-term receivables	3,065	2,510
Energy commodity derivatives contracts (current assets)	13,878	4,914
Forward-starting interest rate swap agreements (noncurrent)	1,008
Energy commodity derivatives contracts (noncurrent liabilities)	(2,008)	(6,457)
Debt	$ (2,426,955)	$ (2,389,700)