BOISE — A divided Idaho Supreme Court has sided with PacifiCorp in the utility's tax fight against the state that officials say will likely shift millions worth of assessments to other classes of property owners.

Justices issued their 3-2 ruling on Monday after months of deliberations, with the majority concluding a 2010 state court judge's decision in favor of PacifiCorp "was not clearly erroneous and was supported by substantial and competent evidence."

In addition to PacifiCorp, whose unit Rocky Mountain Power operates in Idaho, the outcome will also likely effect past tax bills of Idaho Power Co., which has also challenged its assessments in recent years.

At stake is about $11.6 million in disputed payments to counties that now may have to be returned to the utilities.

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In their 32-page decision, a majority of justices decided the Idaho State Tax Commission's chance to dispute the reliability of PacifiCorp's expert witness testimony about the value of its properties — and the resulting tax bill — had come and gone when its lawyers failed to raise the issue when the case was heard two years ago in 4th District Court in Ada County.

In Idaho, operating properties such as power lines, gas pipelines and railroads are assessed centrally by the Tax Commission as a single unit — not separately by each county like real property.

This dispute centered on competing methods for determining the value of a utility's properties.