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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
Mar. 31, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value $ 90,446 $ 132,934
Liabilities measured at fair value 53,354 74,647
Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 27,950 27,947
Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 50,912 94,639
Liabilities measured at fair value 53,354 72,252
Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 11,584 10,348
Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Liabilities measured at fair value   2,395
Fair Value Measurements Level 1
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 39,534 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 [Line Items]    
Assets measured at fair value 27,950 27,947
Fair Value Measurements Level 1 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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 [Line Items]    
Assets measured at fair value 11,584 10,348
Fair Value Measurements Level 1 | Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Liabilities measured at fair value   0
Fair Value Measurements Level 2
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 90,585 130,220
Liabilities measured at fair value 92,816 109,716
Fair Value Measurements Level 2 | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 0 0
Fair Value Measurements Level 2 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 90,585 130,220
Liabilities measured at fair value 92,816 107,321
Fair Value Measurements Level 2 | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 0 0
Fair Value Measurements Level 2 | Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Liabilities measured at fair value   2,395
Fair Value Measurements Level 3
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 122 183
Liabilities measured at fair value 333 695
Fair Value Measurements Level 3 | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 0 0
Fair Value Measurements Level 3 | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 122 183
Liabilities measured at fair value 333 695
Fair Value Measurements Level 3 | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 0 0
Fair Value Measurements Level 3 | Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Liabilities measured at fair value   0
Netting
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value (39,795) [1] (35,764) [1]
Liabilities measured at fair value (39,795) [1] (35,764) [1]
Netting | Restricted deposits
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 0 [1] 0 [1]
Netting | Commodity derivative contracts
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value (39,795) [1] (35,764) [1]
Liabilities measured at fair value (39,795) [1] (35,764) [1]
Netting | Investments
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets measured at fair value 0 [1] 0 [1]
Netting | Interest rate swap
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
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.