Dr. Toby Cosgrove, the longtime CEO of the Cleveland Clinic, announced on Monday he will be stepping down by the end of this year.

A succession process for his replacement has now begun and Cosgrove will stay on at the clinic in an advisory role, according to a news release. The clinic also said his successor will be a practicing physician, keeping up with the system’s long history as a physician-led institution.

“It is an honor and a privilege to be a part of an extraordinary and forward-thinking organization that puts patients at the center of everything we do,” Cosgrove said in statement.

During his nearly 13 years at the helm, Cosgrove led the prestigious institution through widespread expansion. But his tenure has also been marred with controversy, especially recently due to his ties with the Trump administration.

Under his leadership, revenue at the clinic grew from $3.7 billion in 2004 to $8.5 billion in 2016. He oversaw growth by establishing multiple new locations nationwide and internationally, including setting up services in Canada and Abu Dhabi.

The number of physician-scientists nearly doubled under his leadership from 1,800 to 3,400. And total visits to the clinic increased from 2.8 million to 7.1 million.