Southern Company plans to merge with AGL Resources through a $12 billion acquisition expected to complete by the second half of 2016, the company announced.

The acquisition will create the second–largest utility company in the U.S. with about 9 million utility customers in nine states. The combined companies’ 11 regulated electric and natural gas distribution companies will have a generating capacity of around 46,000 megawatts and have a combined service territory spanning 17 states.

The combined company will maintain nearly 200,000 miles of electric transmission and distribution lines and more than 80,000 miles of gas pipelines. The merger should strengthen reliability for customers and improve current and future energy infrastructure development, according to Southern Company.