Exhibit 18.1
July 24, 2012
NorthWestern Corporation
3010 W. 69th Street
Sioux Falls, SD 57108
Dear Sirs/Madams:
At your request, we have read the description included in your Quarterly Report on Form 10-Q to the
Securities and Exchange Commission for the quarter ended June 30, 2012, of the facts relating to the
change in the date of the annual goodwill impairment test from October 1 to April 1. We believe, on
the basis of the facts so set forth and other information furnished to us by appropriate officials of the
Company, that the accounting change described in your Form 10-Q is to an alternative accounting
principle that is preferable under the circumstances.
We have not audited any consolidated financial statements of NorthWestern Corporation and its
consolidated subsidiaries as of any date or for any period subsequent to December 31, 2011. Therefore,
we are unable to express, and we do not express, an opinion on the facts set forth in the above-mentioned
Form 10-Q, on the related information furnished to us by officials of NorthWestern Corporation, or on the
financial position, results of operations, or cash flows of NorthWestern Corporation and its consolidated
subsidiaries as of any date or for any period subsequent to December 31, 2011.
Yours truly,
/s/ DELOITTE & TOUCHE LLP
Minneapolis, Minnesota