Derivatives, Hedges, Financial Instruments and Carbon Credits - Reconciliation of Beginning and Ending Balances for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assets
Beginning balance	$ 42	$ 644
Transfers into Level 3
Transfers out of Level 3
Total realized and unrealized gains (losses) included in earnings	876	1,995	644
Purchases
Issuances
Sales	(827)	(2,597)
Settlements
Ending balance	91	42	644
Total gains (losses) for the period included in earnings attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date	78	42	644
Liabilities
Beginning balance	(42)	(644)
Transfers into Level 3
Transfers out of Level 3
Total realized and unrealized gains (losses) included in earnings	(721)	(1,844)	(644)
Purchases
Issuances
Sales
Settlements	672	2,446
Ending balance	(91)	(42)	(644)
Total gains (losses) for the period included in earnings attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date	$ (78)	$ (42)	$ (644)

Derivatives, Hedges, Financial Instruments and Carbon Credits - Reconciliation of Beginning and Ending Balances for Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $)
In Thousands, unless otherwise specified

12 Months Ended

Dec. 31, 2012

Dec. 31, 2011

Dec. 31, 2010

Assets

Beginning balance

$ 42

$ 644

Transfers into Level 3

Transfers out of Level 3

Total realized and unrealized gains (losses) included in earnings

876

1,995

644

Purchases

Issuances

Sales

(827)

(2,597)

Settlements

Ending balance

644

Total gains (losses) for the period included in earnings attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date

644

Liabilities

Beginning balance

(42)

(644)

Transfers into Level 3

Transfers out of Level 3

Total realized and unrealized gains (losses) included in earnings

(721)

(1,844)

(644)

672

2,446

Ending balance

(91)

(42)

(644)

Total gains (losses) for the period included in earnings attributed to the change in unrealized gains or losses on assets and liabilities still held at the reporting date

$ (78)

$ (42)

$ (644)

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