Derivative Transactions (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Derivative Transactions
Unrealized gains on derivative instruments recognized in other comprehensive income (loss)	$ 106	$ 715
Short-term liability related to derivative instruments included in accrued liabilities	36		112
Natural gas futures contracts
Derivative Transactions
Unrealized gains on derivative instruments recognized in other comprehensive income (loss)	106	715
Derivative liability	2		107
Short-term liability related to derivative instruments included in accrued liabilities	36		112
Short-term assets related to derivative instruments included in prepaid expenses and other current assets	34		5
Loss on derivative instrument recognized in cost of sales	22	1,107
Remaining unrecognized loss recorded as a component of accumulated other comprehensive income (loss)	$ 2
Natural gas futures contracts | April to June, 2013
Notional amounts and weighted-average fixed prices per gasoline gallon equivalent of derivative instruments
Gallons	540,000
Natural gas futures contracts | April to June, 2013 | Weighted Average
Notional amounts and weighted-average fixed prices per gasoline gallon equivalent of derivative instruments
Price Per Gasoline Gallon Equivalent	0.51