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	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]
Beginning balance	$ 2,779	$ 1,284
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total realized and unrealized gains (losses) included in earnings	(30)	1,925
Purchases	0	0
Issuances	0	0
Sales	(33)	(1,536)
Settlements	0	0
Ending balance	2,716	1,673
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		1,004
Beginning balance	(2,779)	(1,284)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Total realized and unrealized gains (losses) included in earnings	30	(1,634)
Purchases	0	0
Issuances	0	0
Sales	0	0
Settlements	33	1,245
Ending balance	(2,716)	(1,673)
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		$ (1,004)

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

3 Months Ended

Mar. 31, 2015

Mar. 31, 2014

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]

Beginning balance

$ 2,779us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

$ 1,284us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

Transfers into Level 3

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3

Transfers out of Level 3

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3

Total realized and unrealized gains (losses) included in earnings

(30)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

1,925us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

Purchases

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases

Issuances

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues

Sales

(33)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales

(1,536)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales

Settlements

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements

Ending balance

2,716us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

1,673us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

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

1,004us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease

Beginning balance

(2,779)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

(1,284)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

Transfers into Level 3

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3

Transfers out of Level 3

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3

Total realized and unrealized gains (losses) included in earnings

30us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings

(1,634)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings

Purchases

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases

Issuances

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues

Sales

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales

Settlements

33us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements

1,245us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements

Ending balance

(2,716)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

(1,673)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

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

$ (1,004)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease

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Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy.

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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