Long-Term Debt (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt	$ 764,056	$ 763,997
3.60% Senior Notes Due 2022
Debt Instrument [Line Items]
Maturity	2022
Stated interest rate	3.60%
Senior notes	$ 249,167	249,108
6 1/2% Senior Notes Due 2029
Debt Instrument [Line Items]
Maturity	2029
Stated interest rate	6.50%
Senior notes	$ 100,000	100,000
6 3/4% Senior Notes Due 2032
Debt Instrument [Line Items]
Maturity	2032
Stated interest rate	6.75%
Senior notes	$ 250,000	250,000
2035 GO Zone 6 1/2% Notes
Debt Instrument [Line Items]
Maturity	2035
Stated interest rate	6.50%
Senior notes	$ 89,000	89,000
2035 IKE Zone 6 1/2% Notes
Debt Instrument [Line Items]
Maturity	2035
Stated interest rate	6.50%
Senior notes	$ 65,000	65,000
Loan related to tax-exempt waste disposal revenue bonds due 2027
Debt Instrument [Line Items]
Maturity	2027
Non-current loan payable	$ 10,889	$ 10,889

Contractual interest rate for funds borrowed, under the debt agreement.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Including the current and noncurrent portions, aggregate carrying value as of the balance sheet date of loans payable (with maturities initially due after one year or beyond the operating cycle if longer).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16(a)(2))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

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Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16(a))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Name Accounting Standards Codification

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Debt Instrument, Maturity

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