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Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value $ 135,935 $ 132,934
Liabilities measured at fair value 13,515 74,647
Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 27,953 27,947
Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 96,597 94,639
Liabilities measured at fair value 13,515 72,252
Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 11,385 10,348
Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value   2,395
Fair Value Measurements Level 1
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 39,338 38,295
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 27,953 27,947
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 11,385 10,348
Fair Value Measurements Level 1 | Interest rate swap
    
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 137,484 130,220
Liabilities measured at fair value 54,402 109,716
Fair Value Measurements Level 2 | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 0
Fair Value Measurements Level 2 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 137,484 130,220
Liabilities measured at fair value 54,402 107,321
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 | Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value   2,395
Fair Value Measurements Level 3
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 183
Liabilities measured at fair value 0 695
Fair Value Measurements Level 3 | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 0
Fair Value Measurements Level 3 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 183
Liabilities measured at fair value 0 695
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 | Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Liabilities measured at fair value   0
Netting
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value (40,887) [1] (35,764) [1]
Liabilities measured at fair value (40,887) [1] (35,764) [1]
Netting | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 [1] 0 [1]
Netting | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value (40,887) [1] (35,764) [1]
Liabilities measured at fair value (40,887) [1] (35,764) [1]
Netting | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis    
Assets measured at fair value 0 [1] 0 [1]
Netting | Interest rate swap
    
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.