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, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Assets [Abstract]
Beginning balance	$ 1,284	$ 91	$ 42
Transfers into Level 3	0	0	0
Transfers out of Level 3	0	0	0
Total realized and unrealized gains (losses) included in earnings	3,089	1,233	876
Purchases	0	0	0
Issuances	0	0	0
Sales	(1,594)	(40)	(827)
Settlements	0	0	0
Ending balance	2,779	1,284	91
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	2,110	1,193	78
Beginning balance	(1,284)	(91)	(42)
Transfers into Level 3	0	0	0
Transfers out of Level 3	0	0	0
Total realized and unrealized gains (losses) included in earnings	(2,799)	(1,233)	(721)
Purchases	0	0	0
Issuances	0	0	0
Sales	0	0	0
Settlements	1,304	40	672
Ending balance	(2,779)	(1,284)	(91)
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	$ (2,110)	$ (1,193)	$ (78)

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, 2014

Dec. 31, 2013

Dec. 31, 2012

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

Beginning balance

$ 1,284us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

$ 91us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

$ 42us-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

3,089us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

1,233us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

876us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1

Purchases

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases

Issuances

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues

Sales

(1,594)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales

(40)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales

(827)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales

Settlements

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements

Ending balance

2,779us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

1,284us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue

91us-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

2,110us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease

1,193us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease

78us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease

Beginning balance

(1,284)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

(91)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

(42)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

(2,799)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings

(1,233)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings

(721)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings

Purchases

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases

Issuances

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues

Sales

0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales

Settlements

1,304us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements

40us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements

672us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements

Ending balance

(2,779)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

(1,284)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue

(91)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

$ (2,110)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease

$ (1,193)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease

$ (78)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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Amount of issuances of 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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