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Long-Term Debt and Capital Leases Other Long-term Debt (Details) (USD $)
12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 31, 2021
Debt Instrument [Line Items]        
Other Long-term Debt $ 450,000,000us-gaap_OtherLongTermDebt      
Investment in New Market Tax Credit Program 18,169,000nwe_InvestmentinNewMarketTaxCreditProgram 0nwe_InvestmentinNewMarketTaxCreditProgram 0nwe_InvestmentinNewMarketTaxCreditProgram  
New Market Tax Credit [Member]        
Debt Instrument [Line Items]        
Other Long-term Debt 27,000,000us-gaap_OtherLongTermDebt
/ us-gaap_DebtInstrumentAxis
= nwe_NewMarketTaxCreditMember
     
New Market Tax Credit Financing [Member]        
Debt Instrument [Line Items]        
Investments 18,200,000us-gaap_Investments
/ us-gaap_LongtermDebtTypeAxis
= nwe_NewMarketTaxCreditFinancingMember
     
Debt Instrument, term 30 years      
Interest rate, stated percentage 1.146%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= nwe_NewMarketTaxCreditFinancingMember
     
Investment in New Market Tax Credit Program 8,800,000nwe_InvestmentinNewMarketTaxCreditProgram
/ us-gaap_LongtermDebtTypeAxis
= nwe_NewMarketTaxCreditFinancingMember
     
Scenario, Forecast [Member] | New Market Tax Credit Financing [Member]        
Debt Instrument [Line Items]        
Debt instrument, forgiveness       $ 7,900,000us-gaap_DebtInstrumentDecreaseForgiveness
/ us-gaap_LongtermDebtTypeAxis
= nwe_NewMarketTaxCreditFinancingMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember