v2.4.0.8
Derivative Instruments (Details) (Cash Flow Hedging [Member], Price Swaps, Natural Gas, USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Jun. 30, 2013
MMcf
Dec. 31, 2010
Derivative Instruments and Hedging Activities Disclosure [Line Items]    
Notional Amount of Outstanding Natural Gas Forward Contracts (in MMcf) 197,700,000  
Unrealized Gain Expected to be Reclassified from Other Comprehensive Income and into Earnings During the Next Twelve Months $ 49,614  
Derivative Assets Designated as Hedging Instruments 120,973 135,969
Derivative Liabilities Designated as Hedging Instruments 4,854 7,024
Prepaid Expenses
    
Derivative Instruments and Hedging Activities Disclosure [Line Items]    
Derivative Assets, Current 82,061 80,057
Other Assets
    
Derivative Instruments and Hedging Activities Disclosure [Line Items]    
Derivative Assets, Noncurrent 38,912 55,912
Other Accrued Liabilities [Member]
    
Derivative Instruments and Hedging Activities Disclosure [Line Items]    
Derivative Assets, Current 860 970
Other Liabilities [Member]
    
Derivative Instruments and Hedging Activities Disclosure [Line Items]    
Derivative Liabilities, Noncurrent $ 3,994 $ 6,054