Fair Value Measurements (Details) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper	$ 64,982,000	$ 84,830,000
Recurring basis | Total
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper	64,982,000	84,830,000
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 | Total | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity contract - financial assets	3,046,000	3,159,000
Commodity contract - financial liabilities	2,299,000	4,378,000
Recurring basis | Total | Commodity options
Assets and liabilities subject to fair value measurements
Commodity contract - financial assets	37,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper	64,982,000	84,830,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity contract - financial assets	3,046,000	3,159,000
Commodity contract - financial liabilities	2,299,000	4,378,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Commodity options
Assets and liabilities subject to fair value measurements
Commodity contract - financial assets	$ 37,000