Fair Value (Carrying Amounts and Fair Values of Our Financial Instruments) (Details) (USD $) In Thousands	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010	Dec. 31, 2009
Cash and cash equivalents, Carrying Amount	$ 12,992	$ 7,483	$ 35,094	$ 4,168
Energy commodity derivatives deposits	43,505	22,302
Long-term receivables	1,710	1,167
Energy commodity derivatives contracts (current)	(8,180)	(11,790)
Energy commodity derivatives contracts (noncurrent)	(10,962)	(4,920)
Debt	(2,042,246)	(1,906,148)
Interest rate swaps (current)	2,678
Interest rate swaps (noncurrent)	(1,849)
Carrying Amount [Member]
Cash and cash equivalents, Carrying Amount	12,992	7,483
Restricted cash		14,379
Energy commodity derivatives deposits	43,505	22,302
Long-term receivables	1,710	1,167
Energy commodity derivatives contracts (current)	(8,180)	(11,790)
Energy commodity derivatives contracts (noncurrent)	(10,962)	(4,920)
Debt	(2,042,246)	(1,906,148)
Interest rate swaps (current)	2,678
Interest rate swaps (noncurrent)	(1,849)
Fair Value [Member]
Cash and cash equivalents, Fair Value	12,992	7,483
Restricted cash		14,379
Energy commodity derivatives deposits	43,505	22,302
Long-term receivables	1,705	1,161
Energy commodity derivatives contracts (current)	(8,180)	(11,790)
Energy commodity derivatives contracts (noncurrent)	(10,962)	(4,920)
Debt, fair value	(2,247,520)	(2,048,895)
Interest rate swaps (current)	2,678
Interest rate swaps (noncurrent)	$ (1,849)

Interest Rate Swaps Current

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Interest Rate Swaps Noncurrent

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Carrying value as of the balance sheet date of revolving line of credit and senior public offering notes, noncurrent.

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Includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits are not generally reported as cash and cash equivalents. Includes cash and cash equivalents associated with the entity's continuing operations. Excludes cash and cash equivalents associated with the disposal group (and discontinued operation).

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.

+ References

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Carrying amount as of the balance sheet date of the asset arising from commodity contracts such as futures contracts tied to the movement of a particular commodity, which are expected to be converted into cash or otherwise disposed of after a year or beyond the normal operating cycle, if longer.

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Fair values as of the balance sheet date for all assets resulting from contracts that meet the criteria of being accounted for as derivative instruments and which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer, net of the effects of master netting arrangements.

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The total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle) and amount due to the Entity from outside sources, including trade accounts receivable, notes and loans receivable, as well as any other types of receivables, net of allowances established for the purpose of reducing such investments and receivables to an amount that approximates their net realizable value.

+ References

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The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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