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Standard & Poors (S&P), one of three corporate and​ municipal body credit rating agencies, upgraded Delta's corporate credit rating to BBB-, investment grade status — giving Delta an investment-grade credit rating at all three agencies.

"This outstanding achievement, which we should all be proud of, was made possible by the great work Delta people do every day around the world to take care of our customers, our shareholders and each other, "said Paul Jacobson, Delta's Chief Financial Officer.

In a report issued Thursday, S&P said that Delta continues to report strong operating performance compared with other global network airlines as well as generating higher margins and returns on capital.

Delta joins Southwest Airlines as the major U.S. carriers to have investment-grade credit ratings from all three agencies. Moody's rating of investment-grade status came in Feb. 2016 with Fitch following in May 2016.