Fair Value Measurements (Details) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2012 item	Dec. 31, 2012	Dec. 31, 2010	Dec. 31, 2011
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		$ 31,520,000		$ 84,830,000
Carrying amount of long-term debt, including current maturities		2,202,237,000		2,380,100,000
Number of Small Joint Ventures Terminated	2
Impairment charges	8,300,000	8,250,000	12,805,000
Assets , fair value	5,800,000
Recurring basis | Total
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		31,520,000		84,830,000
Recurring basis | Total | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets		4,024,000		3,159,000
Commodity futures - financial liabilities		1,854,000		4,378,000
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		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 | Significant Other Observable Inputs (Level 2) | Commodity futures net
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets		4,024,000		3,159,000
Commodity futures - financial liabilities		$ 1,854,000		$ 4,378,000