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Fair Value Measurements Fair Value Measurements - Changes in Fair Value of Risk Management Assets and Liabilities (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Total net gains (losses) realized/unrealized:    
Net derivative beginning balance $ (49,000,000)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs $ (48,000,000)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs
Included in earnings 0us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInEarnings 0us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationGainLossIncludedInEarnings
Included in OCI 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisGainLossIncludedInOtherComprehensiveIncomeLoss 0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisGainLossIncludedInOtherComprehensiveIncomeLoss
Deferred as a regulatory asset or liability 0pnw_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisDerivativeGainLossDeferredAsRegulatoryAssetOrLiability (10,000,000)pnw_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisDerivativeGainLossDeferredAsRegulatoryAssetOrLiability
Settlements 12,000,000us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationSettlements 10,000,000us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationSettlements
Transfers into Level 3 from Level 2 (2,000,000)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationTransfersIntoLevel3 0us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationTransfersIntoLevel3
Transfers from Level 3 into Level 2 (2,000,000)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationTransfersOutOfLevel3 (1,000,000)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisUnobservableInputsReconciliationTransfersOutOfLevel3
Net derivative ending balance (41,000,000)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs (49,000,000)us-gaap_FairValueNetDerivativeAssetLiabilityMeasuredOnRecurringBasisWithUnobservableInputs
Net unrealized gains included in earnings related to instruments still held at end of period 0pnw_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisChangeinUnrealizedGainLoss 0pnw_FairValueNetDerivativeAssetLiabilityMeasuredonRecurringBasisChangeinUnrealizedGainLoss
Significant level 1 transfers $ 0pnw_FairValueMeasurementInterLevelTransfersAmount