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Components of deferred tax assets and liabilities (Details 3) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Valuation Allowance, Amount $ 0us-gaap_DeferredTaxAssetsValuationAllowance $ 0us-gaap_DeferredTaxAssetsValuationAllowance
Deferred tax assets:    
Regulatory matters 134,567,000us-gaap_DeferredTaxAssetsRegulatoryAssetsAndLiabilities 125,607,000us-gaap_DeferredTaxAssetsRegulatoryAssetsAndLiabilities
Accrued pension costs 97,690,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsPensions 74,320,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsPensions
Alternative minimum tax credit carryforward 23,844,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax 33,304,000us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax
Compensation-related 38,654,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeCompensation 31,550,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsEmployeeCompensation
Asset retirement obligations 34,296,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAssetRetirementObligations 29,578,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsAssetRetirementObligations
Legal and environmental contingencies 10,049,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLegalSettlements 10,710,000us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccrualsLegalSettlements
Other 59,389,000us-gaap_DeferredTaxAssetsOther 45,101,000us-gaap_DeferredTaxAssetsOther
Total deferred tax assets 398,489,000us-gaap_DeferredTaxAssetsGross 350,170,000us-gaap_DeferredTaxAssetsGross
Deferred tax liabilities:    
Depreciation and basis differences on property, plant and equipment 906,455,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment 813,597,000us-gaap_DeferredTaxLiabilitiesPropertyPlantAndEquipment
Basis differences on oil and natural gas producing properties 270,939,000us-gaap_DeferredTaxLiabilitiesOtherFiniteLivedAssets 266,168,000us-gaap_DeferredTaxLiabilitiesOtherFiniteLivedAssets
Regulatory matters 97,521,000us-gaap_DeferredTaxLiabilitiesRegulatoryAssetsAndLiabilities 64,914,000us-gaap_DeferredTaxLiabilitiesRegulatoryAssetsAndLiabilities
Intangible asset amortization 22,505,000us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets 13,579,000us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets
Other 29,676,000us-gaap_DeferredTaxLiabilitiesOther 26,170,000us-gaap_DeferredTaxLiabilitiesOther
Total deferred tax liabilities 1,327,096,000us-gaap_DeferredIncomeTaxLiabilities 1,184,428,000us-gaap_DeferredIncomeTaxLiabilities
Net deferred income tax liability $ (928,607,000)us-gaap_DeferredTaxAssetsLiabilitiesNet $ (834,258,000)us-gaap_DeferredTaxAssetsLiabilitiesNet