Commitments and Contingencies Legal Proceedings (Details) (Colstrip Energy Limited Partnership Rate Dispute [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Colstrip Energy Limited Partnership Rate Dispute [Member]
Loss Contingencies [Line Items]
Loss Contingency Allegation	$ 23

Presents an assertion of a fact by a plaintiff in a pleading or complaint, which the plaintiff claims it will prove upon presentation of evidence at the proceeding.

No definition available.

Name:	nwe_LossContingencyAllegation
Namespace Prefix:	nwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant