Ameren (NYSE:AEE) announced today that the Illinois Commerce Commission (ICC) has approved a majority of the construction of a $1.1 billion 400-mile 345-kilovolt line, the utility's largest-ever transmission project.

The project is also the largest single investment by Ameren in around 30 years, and is expected to provide a "significant boost" to Illinois' economy throughout its five-year construction period.

"We are pleased with the ICC's decision confirming that the Illinois Rivers project is necessary and the best approach to addressing the reliability needs of customers and ensuring the development of an efficient electricity market," said President and CEO of Ameren's Illinois transmission business, Maureen Borkowski, in a statement. "This project will help fuel the Illinois economy and provide customers access to a variety of energy sources, including wind energy."

Ameren is moving ahead immediately with construction on approved sections, while it requests rehearings for the two of nine route segments, and six of nine bulk substation locations, that weren't given the initial thumbs up. The ICC listed a lack of time and evidence on the most cost-effective options as main reasons for its pushback, which Ameren will address in the next round of rehearings.