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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

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