"The outlooks on SLM's long-term issuer and senior unsecured debt ratings were revised to negative from stable reflecting high double leverage at the parent company."
https://www.moodys.com/research/Moodys-Ratings-affirms-SLM-Corporations-ratings-Ba1-senior-unsecured-changes-Rating-Action--PR_493009?cid=GAR9PTU7VKT2671