Fair Value Measurements (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Commodity Contract | Futures
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	$ 5,130,000	$ 1,010,000
Commodity futures - financial liabilities	1,225,000	4,487,000
Recurring | Level 2
Assets and liabilities subject to fair value measurements
Fair value of long-term debt, including current maturities	3,100,000,000	2,300,000,000
Carrying amount of long-term debt, including current maturities	3,100,000,000	2,100,000,000
Recurring | Level 2 | Commodity Contract | Futures
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	5,130,000	1,010,000
Commodity futures - financial liabilities	1,225,000	4,487,000
Recurring | Total | Commodity Contract | Futures
Assets and liabilities subject to fair value measurements
Commodity futures - financial assets	5,130,000	1,010,000
Commodity futures - financial liabilities	$ 1,225,000	$ 4,487,000