Derivative Instruments (Details) (Cash Flow Hedging [Member], Price Swaps, Natural Gas, USD $)	3 Months Ended
	Mar. 31, 2013 MMcf	Dec. 31, 2010
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Notional Amount of Outstanding Natural Gas Forward Contracts (in MMcf)	147,400
Unrealized Gain Expected to be Reclassified from Other Comprehensive Income and into Earnings During the Next Twelve Months	$ 20,494,000
Derivative Assets Designated as Hedging Instruments	83,677,000	135,969,000
Derivative Liabilities Designated as Hedging Instruments	20,949,000	7,024,000
Prepaid Expenses
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Assets, Current	46,837,000	80,057,000
Other Assets
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Assets, Noncurrent	36,840,000	55,912,000
Other Accrued Liabilities [Member]
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Assets, Current	10,877,000	970,000
Other Liabilities [Member]
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Liabilities, Noncurrent	$ 10,072,000	$ 6,054,000