For the quarterly period ended June 30, 2019 UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 10-Q (mark OR one) QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 NORTHWESTERN CORP (Exact name of registrant as specified in its charter) Delaware 46-0172280 (State or other jurisdiction of incorporation or organization) (I.R.S. Employer Identification No.) 3010 W. 69th Street Sioux Falls South Dakota 57108 (Address of principal executive offices ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 10-Q (mark one) x QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the quarterly period ended March 31, 2019 OR o TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission File Number: 1-10499 NORTHWESTERN CORPORATION (Exact name of registrant as specified in its charter) | Delaware | 46-0172280 | | --- | --- | | ( ...

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Form 10-K (Mark One) x ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2018 OR o TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from to Commission File Number: 1-10499 NORTHWESTERN CORPORATION (Exact name of registrant as specified in its charter) (State or other jurisdiction of incorporation or ...