Derivative Instruments (Details) (Cash Flow Hedging [Member], Price Swaps, Natural Gas, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Sep. 30, 2013 MMcf	Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Notional Amount of Outstanding Natural Gas Forward Contracts (in MMcf)	216,500,000
Unrealized Gain Expected to be Reclassified from Other Comprehensive Income and into Earnings During the Next Twelve Months	$ 44,438
Derivative Assets Designated as Hedging Instruments	107,366	135,969
Derivative Liabilities Designated as Hedging Instruments	5,252	7,024
Prepaid Expenses
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Assets, Current	75,735	80,057
Other Assets
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Assets, Noncurrent	31,631	55,912
Other Accrued Liabilities [Member]
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Assets, Current	746	970
Other Liabilities [Member]
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Derivative Liabilities, Noncurrent	$ 4,506	$ 6,054