Fitch Ratings cut the debt rating Thursday to A- from A, after putting the utility on watch when it announced the $8 billion purchase last year.
Southern Co. owns Georgia Power Co., Alabama Power Co., Gulf Power Co. and Mississippi Power Co.
Fitch is also cutting the rating of Mississippi Power to BBB from BBB+, saying Southern Co.'s smallest subsidiary still faces "significant stress" because of the $6.7 billion power plant it's building in Kemper County. Fitch says those risks are offset by strong financial support Southern Co. has given to Mississippi Power.
Fitch says AGL Resources will make Southern Co. less risky overall.