Black Hills Corp. and NorthWestern Energy announced Tuesday an agreement to merge in an all-stock, tax-free transaction

Under the all-stock deal, NorthWestern shareholders will receive 0.98 Black Hills share for each held. This per-share value of $59.14 gives NorthWestern an equity value of $3.6 billion, according to Reuters' calculation.

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.