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Income Taxes Schedule of Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Abstract]
Federal statutory tax rate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Federal tax credits	(11.40%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther	(21.80%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther	(11.80%)us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsOther
State and local taxes, net of federal tax benefit	3.90%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	3.40%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes	3.50%us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance	0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	0.00%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance	2.60%us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance
Flow through depreciation and cost basis differences	(2.30%)us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciation	2.80%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciation	2.40%us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseDepreciation
Other	0.80%us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments	(2.60%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments	(0.60%)us-gaap_EffectiveIncomeTaxRateReconciliationOtherAdjustments
Effective tax rate	26.00%us-gaap_EffectiveIncomeTaxRateContinuingOperations	16.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations	31.10%us-gaap_EffectiveIncomeTaxRateContinuingOperations

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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