Fair Value Measurements (Details) (USD $)	Sep. 30, 2011 notes	Dec. 31, 2010
Fair Value Measurements (Details Textual)
Number of notes receivable	2
Notes receivables, carrying value	$ 130,500,000
Notes receivable, fair value	151,000,000
Total Debt outstanding	1,070,702,000	1,159,215,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value assets measured on recurring basis [Abstract]
Deferred compensation plan Investments	13,422,000
Total assets measured at fair value	13,422,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair value liabilities measured on recurring basis [Abstract]
Variable to fixed natural gas swaps	345,000
Total liabilities measured at fair value	345,000
Fair Value, Inputs, Level 1 [Member]
Fair value assets measured on recurring basis [Abstract]
Deferred compensation plan Investments	13,422,000
Total assets measured at fair value	13,422,000
Fair Value, Inputs, Level 1 [Member]
Fair value liabilities measured on recurring basis [Abstract]
Variable to fixed natural gas swaps	0
Total liabilities measured at fair value	0
Fair Value, Inputs, Level 2 [Member]
Fair value assets measured on recurring basis [Abstract]
Deferred compensation plan Investments	0
Total assets measured at fair value	0
Fair Value, Inputs, Level 2 [Member]
Fair value liabilities measured on recurring basis [Abstract]
Variable to fixed natural gas swaps	345,000
Total liabilities measured at fair value	345,000
Fair Value, Inputs, Level 3 [Member]
Fair value assets measured on recurring basis [Abstract]
Deferred compensation plan Investments	0
Total assets measured at fair value	0
Fair Value, Inputs, Level 3 [Member]
Fair value liabilities measured on recurring basis [Abstract]
Variable to fixed natural gas swaps	0
Total liabilities measured at fair value	0
Convertible Notes Payable [Member]
Fair Value Measurements (Details Textual)
Total Debt outstanding	315,900,000
Face amount of outstanding debt	360,000,000
Accrued interest on convertible notes	3.75%
Fair value of the Convertible Notes	330,000,000
Senior Notes [Member]
Fair Value Measurements (Details Textual)
Total Debt outstanding	152,200,000
Accrued interest on convertible notes	6.75%
Fair value of senior notes	$ 147,600,000

Fair value assets measured on recurring basis total assets.

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Fair value assets measured on recurring basis.

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Fair value assets measured on recurring basis deferred compensation plan investments.

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Fair value liabilities measured on recurring basis total liabilities.

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Fair value liabilities measured on recurring basis.

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Fair value liabilities measured on recurring basis variable to fixed natural gas swaps.

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Fair Value Measurements Details Textual

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Number of notes receivable.

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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. Fair value of convertible debt at the balance sheet date.

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Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

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The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

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

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

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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.

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

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-Name Regulation S-X (SX)

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-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

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-Name Regulation S-X (SX)

-Number 210

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