Fair Value Measurements (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		$ 31,520,000
Carrying amount of long-term debt, including current maturities	2,107,589,000	2,202,237,000
Recurring basis | Total
Assets and liabilities subject to fair value measurements
Investments in short-term commercial paper		31,520,000
Carrying amount of long-term debt, including current maturities	2,100,000,000	2,200,000,000
Recurring basis | Total | Commodity contract | Futures
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets	1,010,000	4,024,000
Commodity futures - financial liabilities	4,487,000	1,854,000
Recurring basis | Level 2
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	2,300,000,000	2,300,000,000
Recurring basis | Level 2 | Commodity contract | Futures
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets	1,010,000	4,024,000
Commodity futures - financial liabilities	$ 4,487,000	$ 1,854,000