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Goodwill (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Goodwill [Line Items]    
Goodwill $ 355,128us-gaap_Goodwill $ 355,128us-gaap_Goodwill
Goodwill, Period Increase (Decrease) 0us-gaap_GoodwillPeriodIncreaseDecrease 0us-gaap_GoodwillPeriodIncreaseDecrease
Electric    
Goodwill [Line Items]    
Goodwill 241,100us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= nwe_ElectricDomesticRegulatedMember		 241,100us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= nwe_ElectricDomesticRegulatedMember
Natural gas    
Goodwill [Line Items]    
Goodwill $ 114,028us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= nwe_GasDomesticRegulatedMember		 $ 114,028us-gaap_Goodwill
/ us-gaap_StatementBusinessSegmentsAxis
= nwe_GasDomesticRegulatedMember