Income Taxes Deferred Tax Liability (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred Tax Assets, [Abstract]
Pension / postretirement benefits	$ 51,817	$ 20,522
NOL carryforward	42,787	16,758
Unbilled revenue	19,863nwe_DeferredTaxAssetsUnbilledRevenue	18,136nwe_DeferredTaxAssetsUnbilledRevenue
Compensation accruals	17,315	10,409
Customer advances	11,817nwe_DeferredTaxAssetsCustomerAdvances	10,781nwe_DeferredTaxAssetsCustomerAdvances
AMT credit carryforwards	10,357	10,357
Environmental liability	8,968	9,026
Production tax credits	6,452	3,171
Interest rate hedges	6,251	0
QF obligations	2,162nwe_DeferredTaxAssetQfObligation	2,066nwe_DeferredTaxAssetQfObligation
Reserves and accruals	1,772	12,097
Property taxes	881nwe_DeferredTaxAssetPropertyTaxes	796nwe_DeferredTaxAssetPropertyTaxes
Regulatory liabilities	975	659
Regulatory assets	0	7,248
Other, net	4,415us-gaap_DeferredTaxAssetsOther	2,827us-gaap_DeferredTaxAssetsOther
Deferred Tax Asset	185,832us-gaap_DeferredTaxAssetsGross	124,853us-gaap_DeferredTaxAssetsGross
Deferred Tax Liabilities, [Abstract]
Excess tax depreciation	(349,428)	(304,071)
Goodwill amortization	(137,090)	(122,798)
Flow through depreciation	(103,677)	(79,016)
Regulatory assets	(21,394)	0
Deferred Tax Liability	(611,589)us-gaap_DeferredTaxLiabilities	(505,885)us-gaap_DeferredTaxLiabilities
Deferred Tax Liability, net	$ (425,757)us-gaap_DeferredTaxAssetsLiabilitiesNet	$ (381,032)us-gaap_DeferredTaxAssetsLiabilitiesNet

Deferred Tax Asset Property Taxes

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Deferred tax asset associated with our Qualifying Facilities (QF) obligations

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Deferred Tax Assets Customer Advances

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- Definition

Deferred Tax Assets Environmental Liability

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Deferred tax assets resulting from qualified production activities

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences derived regulatory liability.

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Deferred Tax Assets Unbilled Revenue

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Deferred Tax Liabilities Excess Tax Depreciation

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Deferred Tax Liabilities Flow Through Depreciation

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from hedging transactions.

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Amount after allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences and carryforwards, net of deferred tax liability attributable to taxable temporary differences.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible operating loss carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences not separately disclosed.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences derived regulatory assets and liabilities.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible alternative minimum tax credit carryforwards.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from other compensation and benefits not separately disclosed.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from pension benefits.

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Amount before allocation of valuation allowances of deferred tax asset attributable to deductible temporary differences from reserves and accruals.

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Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.

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Amount of deferred tax liability attributable to taxable temporary differences from goodwill.

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Amount of deferred tax liability attributable to taxable temporary differences from regulatory assets and liabilities.

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