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NorthWestern Energy announced Tuesday it plans to merge with Black Hills Corp “to create a premier regional regulated electric and natural gas utility company” in a process expected to take 12 to 15 months and result in an enterprise valued at $15.4 billion.

NorthWestern Energy is a monopoly utility that operates in Montana, South Dakota and Nebraska and serves 787,000 customers. It counts 413,400 electric and 214,500 natural gas customers in Montana.

Black Hills is based in South Dakota and serves 1.35 million natural gas and electric utility customers in eight states, the news release said. It operates in Arkansas, Colorado, Iowa, Kansas, Montana, Nebraska, South Dakota and Wyoming.

The announcement from NorthWestern Energy said the boards of dir

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