Fair value measurements (Details) (USD $)	3 Months Ended	9 Months Ended
Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Assets [Abstract]
Money market funds	$ 56,194,000	$ 2,835,000	$ 56,194,000	$ 2,835,000	$ 166,620,000
Available-for-sale securities [Abstract]
Fixed-income securities		11,400,000		11,400,000	11,400,000
Insurance investment contract	33,591,000	35,902,000	33,591,000	35,902,000	39,541,000
Auction rate securities	11,400,000		11,400,000
Mortgage-backed securities	8,570,000		8,570,000
U.S. Treasury securities	1,444,000		1,444,000
Commodity derivative instruments - current asset	38,794,000	26,803,000	38,794,000	26,803,000	15,123,000
Commodity derivative instruments - noncurrent asset	15,575,000	8,423,000	15,575,000	8,423,000	4,104,000
Total assets measured at fair value	165,568,000	85,363,000	165,568,000	85,363,000	236,788,000
Liabilities [Abstract]
Commodity derivative instruments - current liability	3,028,000	25,803,000	3,028,000	25,803,000	24,428,000
Commodity derivative instruments - noncurrent liability	157,000	2,263,000	157,000	2,263,000	6,483,000
Interest rate derivative instruments - noncurrent	3,491,000		3,491,000
Total liabilities measured at fair value	6,676,000	28,066,000	6,676,000	28,066,000	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	33,600,000	35,900,000	33,600,000	35,900,000	39,500,000
Before tax amount of increase (decrease) in fair value of investments used to satisfy obligations under nonqualified benefit plans	(6,700,000)	3,200,000	(5,900,000)	2,200,000
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	2,835,000	0	2,835,000	0
Available-for-sale securities [Abstract]
Fixed-income securities		0		0	0
Insurance investment contract	0	0	0	0	0
Auction rate securities	0		0
Mortgage-backed securities	0		0
U.S. Treasury securities	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	2,835,000	0	2,835,000	0
Liabilities [Abstract]
Commodity derivative instruments - current liability	0	0	0	0	0
Commodity derivative instruments - noncurrent liability	0	0	0	0	0
Interest rate derivative instruments - noncurrent	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	56,194,000	0	56,194,000	0	166,620,000
Available-for-sale securities [Abstract]
Fixed-income securities		11,400,000		11,400,000	11,400,000
Insurance investment contract	33,591,000	35,902,000	33,591,000	35,902,000	39,541,000
Auction rate securities	11,400,000		11,400,000
Mortgage-backed securities	8,570,000		8,570,000
U.S. Treasury securities	1,444,000		1,444,000
Commodity derivative instruments - current asset	38,794,000	26,803,000	38,794,000	26,803,000	15,123,000
Commodity derivative instruments - noncurrent asset	15,575,000	8,423,000	15,575,000	8,423,000	4,104,000
Total assets measured at fair value	165,568,000	82,528,000	165,568,000	82,528,000	236,788,000
Liabilities [Abstract]
Commodity derivative instruments - current liability	3,028,000	25,803,000	3,028,000	25,803,000	24,428,000
Commodity derivative instruments - noncurrent liability	157,000	2,263,000	157,000	2,263,000	6,483,000
Interest rate derivative instruments - noncurrent	3,491,000		3,491,000
Total liabilities measured at fair value	6,676,000	28,066,000	6,676,000	28,066,000	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
Insurance investment contract	0	0	0	0	0
Auction rate securities	0		0
Mortgage-backed securities	0		0
U.S. Treasury securities	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
Interest rate derivative instruments - noncurrent	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	1,423,614,000	1,510,588,000	1,423,614,000	1,510,588,000	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	$ 1,568,942,000	$ 1,679,979,000	$ 1,568,942,000	$ 1,679,979,000	$ 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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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 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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Fair values as of the balance sheet date of all liabilities resulting from interest rate swap 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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Investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees

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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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