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Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2012 | Dec. 31, 2011 | Jun. 30, 2011
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Concentration risks, percentage [Abstract] |  |  |  |  |  |
Percentage investment in common stock of mid-cap companies | 28.00% |  | 33.00% |  | 34.00% |
Percentage investment in common stock of small-cap companies | 28.00% |  | 34.00% |  | 33.00% |
Percentage investment in common stock of large-cap companies | 29.00% |  | 32.00% |  | 32.00% |
Percentage in fixed-income and other investments | 15.00% |  |  |  |  |
Percentage investment in cash and cash equivalents |  |  | 1.00% |  | 1.00% |
Fair value, measurements, recurring [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 125,917 |  | 187,659 |  | 90,655 |
Liabilities, fair value disclosure | 8,000 |  | 18,863 |  | 25,421 |
Fair value, measurements, recurring [Member] | Commodity derivative instruments, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 1,037 |  | 14,101 |  | 25,421 |
Fair value, measurements, recurring [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 6,963 |  | 4,762 |  |  |
Fair value, measurements, recurring [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 21,054 |  | 97,500 |  | 8,297 |
Fair value, measurements, recurring [Member] | Insurance investment contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 45,959 | [1] | 38,352 | [2] | 40,328 | [3]
Fair value, measurements, recurring [Member] | Auction rate securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure |  |  | 11,400 |  | 11,400 |
Fair value, measurements, recurring [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 8,253 |  | 8,296 |  | 8,162 |
Fair value, measurements, recurring [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 1,994 |  | 1,656 |  | 1,969 |
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 48,657 |  | 30,455 |  | 20,499 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  | 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance investment contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] | 0 | [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Auction rate securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure |  |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 125,917 |  | 187,659 |  | 90,655 |
Liabilities, fair value disclosure | 8,000 |  | 18,863 |  | 25,421 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 1,037 |  | 14,101 |  | 25,421 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 6,963 |  | 4,762 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 21,054 |  | 97,500 |  | 8,297 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance investment contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 45,959 | [1] | 38,352 | [2] | 40,328 | [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Auction rate securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure |  |  | 11,400 |  | 11,400 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 8,253 |  | 8,296 |  | 8,162 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 1,994 |  | 1,656 |  | 1,969 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 48,657 |  | 30,455 |  | 20,499 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments, liabilities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Interest rate swap [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Liabilities, fair value disclosure | 0 |  | 0 |  |  |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance investment contract [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 | [1] | 0 | [2] | 0 | [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Auction rate securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure |  |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Collateralized mortgage backed securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments , assets [Member] |  |  |  |  |  |
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items] |  |  |  |  |  |
Assets, fair value disclosure | 0 |  | 0 |  | 0 |
[1] | The insurance investment contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other investments.
[2] | The insurance investment contract invests approximately 33 percent in common stock of mid-cap companies, 34 percent in common stock of small-cap companies, 32 percent in common stock of large-cap companies and 1 percent in cash and cash equivalents.
[3] | The insurance investment contract invests approximately 34 percent in common stock of mid-cap companies, 33 percent in common stock of small-cap companies, 32 percent in common stock of large-cap companies and 1 percent in cash and cash equivalents.

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