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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) (Fair Value, Measurements, Recurring, USD $)
In Thousands, unless otherwise specified
Mar. 31, 2014
Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value $ 39,080 $ 68,568
Liabilities measured at fair value 23,655 54,831
Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value   27,955
Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 27,050 26,905
Liabilities measured at fair value 14,175 54,831
Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 12,030 13,708
Guarantees
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value 9,480  
Fair Value Measurements Level 1
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 12,030 41,663
Liabilities measured at fair value 0 0
Fair Value Measurements Level 1 | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 27,955
Fair Value Measurements Level 1 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 0
Liabilities measured at fair value 0 0
Fair Value Measurements Level 1 | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 12,030 13,708
Fair Value Measurements Level 1 | Guarantees
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value 0  
Fair Value Measurements Level 2
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 30,803 50,274
Liabilities measured at fair value 17,928 78,200
Fair Value Measurements Level 2 | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value   0
Fair Value Measurements Level 2 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 30,803 50,274
Liabilities measured at fair value 17,928 78,200
Fair Value Measurements Level 2 | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 0
Fair Value Measurements Level 2 | Guarantees
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value 0  
Fair Value Measurements Level 3
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 0
Liabilities measured at fair value 9,480 0
Fair Value Measurements Level 3 | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value   0
Fair Value Measurements Level 3 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 0
Liabilities measured at fair value 0 0
Fair Value Measurements Level 3 | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 0
Fair Value Measurements Level 3 | Guarantees
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value 9,480  
Netting
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value (3,753) [1] (23,369) [1]
Liabilities measured at fair value (3,753) [1] (23,369) [1]
Netting | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value   0 [1]
Netting | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value (3,753) [1] (23,369) [1]
Liabilities measured at fair value (3,753) [1] (23,369) [1]
Netting | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 [1] 0 [1]
Netting | Guarantees
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value $ 0 [1]  
[1] Represents the impact of netting assets and liabilities with counterparties with which the right of offset exists.