U.N. Ambassador Nikki Haley announced her departure from the Trump administration Wednesday, writing in a resignation letter that she would stay on until Jan. 2019, though the president is expected to find her replacement within the next two to three weeks.

President Trump congratulated Haley for her work, thanking her “on behalf of the country” for “a for great job,” adding, "She's done a fantastic job and we've done a fantastic job together.”

Haley’s departure after 19 months as the U.S. envoy to the U.N., seems unusual in an administration marked with dramatic layoffs, exits and angry tweets, said national security expert Juliette Kayyem on Boston Public Radio Wednesday. But, she said, the circumstances of Haley’s departure aren’t abnormal — the average length of stay for ambassadors is less than two and a half years.

“We’re not used to people leaving the Trump administration without an indictment, without a tweet, without an investigation," Kayyem said. “I think we’re all kind of taken aback by the normalcy, [but if] the cabinet secretary leaves after two years, that is not a story.”