Fair value measurements (Details) (USD $)	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Assets [Abstract]
Money market funds	$ 8,297,000	$ 8,251,000	$ 8,297,000	$ 8,251,000	$ 166,620,000
Available-for-sale securities [Abstract]
Fixed-income securities		11,400,000		11,400,000	11,400,000
Insurance investment contract	40,328,000	20,236,000	40,328,000	20,236,000	39,541,000
Auction rate securities	11,400,000		11,400,000
Mortgage-backed securities	8,162,000		8,162,000
U.S. Treasury securities	1,969,000		1,969,000
Commodity derivative instruments - current asset	14,234,000	24,932,000	14,234,000	24,932,000	15,123,000
Commodity derivative instruments - noncurrent asset	6,265,000	8,524,000	6,265,000	8,524,000	4,104,000
Total assets measured at fair value	90,655,000	73,343,000	90,655,000	73,343,000	236,788,000
Liabilities [Abstract]
Commodity derivative instruments - current liability	18,686,000	20,160,000	18,686,000	20,160,000	24,428,000
Commodity derivative instruments - noncurrent liability	6,735,000	698,000	6,735,000	698,000	6,483,000
Total liabilities measured at fair value	25,421,000	20,858,000	25,421,000	20,858,000	30,911,000
Insurance investment contract investment percentages [Abstract]
Percentage investment in common stock of mid-cap companies (in hundredths)	34.00%		34.00%		35.00%
Percentage investment in common stock of small-cap companies (in hundredths)	33.00%		33.00%		33.00%
Percentage investment in common stock of large-cap companies (in hundredths)	32.00%		32.00%		31.00%
Percentage investment in cash and cash equivalents (in hundredths)	1.00%		1.00%		1.00%
Financial liabilities, balance sheet groupings [Abstract]
Investments used to satisfy obligations under nonqualified benefit plans	40,300,000	20,200,000	40,300,000	20,200,000	39,500,000
Before tax amount of increase (decrease) in fair value of investments used to satisfy obligations under nonqualified benefit plans	(1,300,000)	(1,800,000)	790,000	(970,000)
Auction rate securities	11,400,000	11,400,000	11,400,000	11,400,000	11,400,000
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 1 [Member]
Assets [Abstract]
Money market funds	0	8,251,000	0	8,251,000	0
Available-for-sale securities [Abstract]
Fixed-income securities	0	0	0	0	0
Equity securities	0	0	0	0	0
Insurance investment contract	0	0	0	0	0
Auction rate securities	0	0	0	0	0
Mortgage-backed securities	0	0	0	0	0
U.S. Treasury securities	0	0	0	0	0
Commodity derivative instruments - current asset	0	0	0	0	0
Commodity derivative instruments - noncurrent asset	0	0	0	0	0
Total assets measured at fair value	0	8,251,000	0	8,251,000	0
Liabilities [Abstract]
Commodity derivative instruments - current liability	0	0	0	0	0
Commodity derivative instruments - noncurrent liability	0	0	0	0	0
Total liabilities measured at fair value	0	0	0	0	0
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 2 [Member]
Assets [Abstract]
Money market funds	8,297,000	0	8,297,000	0	166,620,000
Available-for-sale securities [Abstract]
Fixed-income securities	0	11,400,000	0	11,400,000	11,400,000
Equity securities	0	0	0	0	0
Insurance investment contract	40,328,000	20,236,000	40,328,000	20,236,000	39,541,000
Auction rate securities	11,400,000	0	11,400,000	0	0
Mortgage-backed securities	8,162,000	0	8,162,000	0	0
U.S. Treasury securities	1,969,000	0	1,969,000	0	0
Commodity derivative instruments - current asset	14,234,000	24,932,000	14,234,000	24,932,000	15,123,000
Commodity derivative instruments - noncurrent asset	6,265,000	8,524,000	6,265,000	8,524,000	4,104,000
Total assets measured at fair value	90,655,000	65,092,000	90,655,000	65,092,000	236,788,000
Liabilities [Abstract]
Commodity derivative instruments - current liability	18,686,000	20,160,000	18,686,000	20,160,000	24,428,000
Commodity derivative instruments - noncurrent liability	6,735,000	698,000	6,735,000	698,000	6,483,000
Total liabilities measured at fair value	25,421,000	20,858,000	25,421,000	20,858,000	30,911,000
Fair Value, Measurements, Recurring [Member] \| Fair Value, Inputs, Level 3 [Member]
Assets [Abstract]
Money market funds	0	0	0	0	0
Available-for-sale securities [Abstract]
Fixed-income securities	0	0	0	0	0
Equity securities	0	0	0	0	0
Insurance investment contract	0	0	0	0	0
Auction rate securities	0	0	0	0	0
Mortgage-backed securities	0	0	0	0	0
U.S. Treasury securities	0	0	0	0	0
Commodity derivative instruments - current asset	0	0	0	0	0
Commodity derivative instruments - noncurrent asset	0	0	0	0	0
Total assets measured at fair value	0	0	0	0	0
Liabilities [Abstract]
Commodity derivative instruments - current liability	0	0	0	0	0
Commodity derivative instruments - noncurrent liability	0	0	0	0	0
Total liabilities measured at fair value	0	0	0	0	0
Carrying (Reported) Amount, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	1,432,105,000	1,581,265,000	1,432,105,000	1,581,265,000	1,506,752,000
Estimate of Fair Value, Fair Value Disclosure [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	$ 1,550,592,000	$ 1,718,477,000	$ 1,550,592,000	$ 1,718,477,000	$ 1,621,184,000

Securities that are debt instruments that typically have long-term nominal maturities for which the interest rate is reset through an auction process.

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Represents the portion of the balance sheet assertion valued at fair value by the entity in connection with the fair value disclosures required in the footnotes to the financial statements. This item represents available-for-sale securities that are debt instruments (corporate or municipal bonds) which typically have long-term nominal maturities for which the interest rate is reset through an auction process.

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Before tax amount of increase (decrease) in fair value of investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees

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Fair values as of the balance sheet date for all assets resulting from commodity 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.

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Fair values as of the balance sheet date of all assets resulting from commodity contracts that meet the criteria of being accounted for as derivative instruments which are expected to exist longer than one year or beyond the normal operating cycle, if longer.

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Fair values as of the balance sheet date of all liabilities resulting from commodity contracts that meet the criteria of being accounted for as derivative instruments, and which are expected to be extinguished or otherwise disposed of after one year or beyond the normal operating cycle, if longer.

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Represents the portion of the balance sheet assertion valued at fair value by the entity in connection with the fair value disclosures required in the footnotes to the financial statements. This item represents available-for-sale equity securities. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt.

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Represents the portion of the balance sheet assertion valued at fair value by the entity in connection with the fair value disclosures required in the footnotes to the financial statements. This item represents available-for-sale debt securities that provide a return in the form of fixed periodic payments and eventual return of principal at maturity.

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Investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees

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Represents the portion of the balance sheet assertion valued at fair value by the entity in connection with the fair value disclosures required in the footnotes to the financial statements. This item represents investments in short-term money-market instruments which are highly liquid and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates.

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The percentage of insurance investment contracts in cash and cash equivalents.

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The percentage of insurance investment contracts in common stock of large-cap companies.

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The percentage of insurance investment contracts in common stock of mid-cap companies.

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The percentage of insurance investment contracts in common stock of small-cap companies.

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Represents the portion of the balance sheet assertion valued at fair value by the entity in connection with the fair value disclosures required in the footnotes to the financial statements. This item represents available-for-sale debt securities issued by the United States Department of the Treasury and backed by the United States government. Such securities primarily consist of treasury bills, treasury notes, and treasury bonds.

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This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=6947722&loc=d3e13476-108611

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

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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 represents securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages) as of the balance sheet date which have been categorized as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher AICPA
-Name Audit and Accounting Guide (AAG)
-Number AAG-DEP
-Chapter 7
-Paragraph 15
-IssueDate 2006-05-01
-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Mortgage-Backed Securities
-URI http://asc.fasb.org/extlink&oid=6517959

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