Fair Value Measurements (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		$ 31,520,000
Recurring basis | Total
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		31,520,000
Fair value of long-term debt, including current maturities	2,400,000,000	2,300,000,000
Carrying amount of long-term debt, including current maturities	2,300,000,000	2,200,000,000
Recurring basis | Total | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets	5,713,000	4,024,000
Commodity futures - financial liabilities	848,000	1,854,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		31,520,000
Recurring basis | Significant Other Observable Inputs (Level 2) | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets	5,713,000	4,024,000
Commodity futures - financial liabilities	$ 848,000	$ 1,854,000