Fair Value Measurements (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		$ 84,830,000
Recurring basis | Total
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		84,830,000
Fair value of long-term debt, including current maturities	2,300,000,000	2,500,000,000
Carrying amount of long-term debt, including current maturities	2,200,000,000	2,400,000,000
Recurring basis | Total | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity contract - financial assets	2,357,000	3,159,000
Commodity contract - financial liabilities	9,809,000	4,378,000
Recurring basis | Total | Commodity options
Assets and liabilities subject to fair value measurements
Commodity contract - financial liabilities	35,000
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
Recurring basis | Significant Other Observable Inputs (Level 2) | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity contract - financial assets	2,357,000	3,159,000
Commodity contract - financial liabilities	9,809,000	4,378,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Commodity options
Assets and liabilities subject to fair value measurements
Commodity contract - financial liabilities	$ 35,000