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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

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