Fair Value Measurements (Details) (Recurring basis, USD $)	Mar. 31, 2014	Dec. 31, 2013
Total
Assets and liabilities subject to fair value measurements
Carrying amount of long-term debt, including current maturities	$ 2,100,000,000	$ 2,100,000,000
Total | Commodity contract | Futures
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets	3,620,000	1,010,000
Commodity futures - financial liabilities	(1,526,000)	4,487,000
Level 2
Assets and liabilities subject to fair value measurements
Fair value of long-term debt, including current maturities	2,200,000,000	2,300,000,000
Level 2 | Commodity contract | Futures
Assets and liabilities subject to fair value measurements
Commodity futures and options - financial assets	3,620,000	1,010,000
Commodity futures - financial liabilities	$ (1,526,000)	$ 4,487,000