HELENA, Mont. -

NorthWestern Energy says it has received approval from the Federal Trade Commission for its proposed $900 million purchase of 11 hydroelectric dams in Montana.

The company issued a statement Tuesday saying it's a significant step, but it still must be reviewed and approved by the state Public Service Commission and the Federal Energy Regulatory Commission. 

NorthWestern is the largest electric and natural gas utility in Montana. The company announced last September that it reached an agreement to buy 11 hydroelectric dams and a storage dam from PPL Montana.

The company believes the dams will provide an affordable, long-term supply of electricity for customers. In its proposal before the PSC, the company says the purchase will raise electric rates about 4 percent.