Black Hills and NorthWestern Energy said on Tuesday they would merge, creating a $15.4 billion regulated electric and natural gas utility better equipped to invest in grid infrastructure and meet rising U.

Rapid City, S.D.-based Black Hills Corp. and Sioux Falls, S.D.-based NorthWestern Energy Group Inc. will merge in an all-stock deal, forming a publicly traded electric and natural gas utility

NorthWestern Energy Group and Black Hills have agreed to combine to form a large $7.8 billion regulated electric and natural gas utility company at a time of climbing energy demand across the U.S.