Fair Value Measurements (Details) (USD $)	12 Months Ended
	Dec. 31, 2010	Dec. 31, 2011
Assets and liabilities subject to fair value measurements
Investment in short-term commercial paper		$ 84,830,000
Carrying amount of long-term debt, including current maturities	2,386,821,000	2,380,100,000
Impairment charges	12,805,000
Assets , fair value	9,200,000
Recurring basis | Total
Assets and liabilities subject to fair value measurements
Investment in short-term commercial paper		84,830,000
Commodity futures - financial assets	7,052,000	3,159,000
Commodity futures - financial liabilities	12,040,000	4,378,000
Fair value of long-term debt, including current maturities	2,500,000,000	2,500,000,000
Carrying amount of long-term debt, including current maturities	2,400,000,000	2,400,000,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		84,830,000
Commodity futures - financial assets	7,052,000	3,159,000
Commodity futures - financial liabilities	$ 12,040,000	$ 4,378,000