Significant Events Significant Events Note	12 Months Ended
Dec. 31, 2012
Significant events related to regulated operations [Abstract]
Schedule of Significant Events	Significant Events MSTI Impairment The MSTI line is a proposed 500 kV transmission project from southwestern Montana to southeastern Idaho with a potential capacity of 1,500 MW. We previously disclosed that there was significant market uncertainty related to the project, and that we would consider writing down or writing off the costs of the MSTI project depending on the likelihood of reaching an agreement with the Bonneville Power Administration (BPA) to serve its southern Idaho loads. On October 2, 2012, BPA notified us that it had ranked other options ahead of MSTI to serve BPA's southern Idaho loads. This notification was in conjunction with the January 2012 Memorandum of Understanding between NorthWestern and BPA agreeing to explore the potential for MSTI to accommodate BPA's needs. Based on BPA's decision, continued market uncertainty, and permitting issues causing timeline delays, we determined that we will not further pursue development of MSTI at this time. As a result, during the third quarter of 2012 we recorded an impairment charge of substantially all of the capitalized preliminary survey and investigative costs related to MSTI, totaling approximately $24.0 million. Colstrip Energy Limited Partnership CELP is a QF with which we have a power purchase agreement (PPA) for approximately 306,600 MWH's annually through June 2024. Under the terms of the PPA with CELP, energy and capacity rates were fixed for the first fifteen years and beginning July 1, 2004, through the end of the contract, energy and capacity rates are to be determined each year pursuant to a formula, subject to annual review and approval by the MPSC. The MPSC's last final order covered rates through June 30, 2006. CELP filed a complaint against us and the MPSC in Montana district court in 2007, which contested the MPSC's orders. For further discussion of this litigation see Note 19 - Commitments and Contingencies. On November 1, 2012, an arbitration panel issued a final award in our favor. The final award confirmed that the rate methodology used by us for calculating the rates for the July 1, 2006 to June 30, 2011 period was consistent with the PPA and a previous final award issued by the same arbitration panel on October 30, 2009. Based on the clarity provided by the final award regarding the rate calculation for 2006 through the remainder of the PPA, we have updated the calculation of our QF liability and recorded a pre-tax gain of $47.9 million within cost of sales in the Consolidated Income Statements during the fourth quarter of 2012.

Significant Events

MSTI Impairment

The MSTI line is a proposed 500 kV transmission project from southwestern Montana to southeastern Idaho with a potential capacity of 1,500 MW. We previously disclosed that there was significant market uncertainty related to the project, and that we would consider writing down or writing off the costs of the MSTI project depending on the likelihood of reaching an agreement with the Bonneville Power Administration (BPA) to serve its southern Idaho loads. On October 2, 2012, BPA notified us that it had ranked other options ahead of MSTI to serve BPA's southern Idaho loads. This notification was in conjunction with the January 2012 Memorandum of Understanding between NorthWestern and BPA agreeing to explore the potential for MSTI to accommodate BPA's needs. Based on BPA's decision, continued market uncertainty, and permitting issues causing timeline delays, we determined that we will not further pursue development of MSTI at this time. As a result, during the third quarter of 2012 we recorded an impairment charge of substantially all of the capitalized preliminary survey and investigative costs related to MSTI, totaling approximately $24.0 million.

Colstrip Energy Limited Partnership

CELP is a QF with which we have a power purchase agreement (PPA) for approximately 306,600 MWH's annually through June 2024. Under the terms of the PPA with CELP, energy and capacity rates were fixed for the first fifteen years and beginning July 1, 2004, through the end of the contract, energy and capacity rates are to be determined each year pursuant to a formula, subject to annual review and approval by the MPSC. The MPSC's last final order covered rates through June 30, 2006. CELP filed a complaint against us and the MPSC in Montana district court in 2007, which contested the MPSC's orders. For further discussion of this litigation see Note 19 - Commitments and Contingencies.

On November 1, 2012, an arbitration panel issued a final award in our favor. The final award confirmed that the rate methodology used by us for calculating the rates for the July 1, 2006 to June 30, 2011 period was consistent with the PPA and a previous final award issued by the same arbitration panel on October 30, 2009. Based on the clarity provided by the final award regarding the rate calculation for 2006 through the remainder of the PPA, we have updated the calculation of our QF liability and recorded a pre-tax gain of $47.9 million within cost of sales in the Consolidated Income Statements during the fourth quarter of 2012.