According to a release, the transition is expected to take effect at the end of September, when Fossel completes five years in the position.
Ryan, 67, has previously served as president, director and chief executive officer (CEO) of TD Banknorth Inc. and currently serves as a director of Wellpoint, Inc., according to the release.
"I am pleased to have been chosen by my fellow board members to assume the position of chairman," Ryan said in a statement. "Unum is an exceptional organization with a track record of outstanding performance, and I look forward to working with our strong board and management team to continue to create value for our shareholders."
The new non-executive appointment comes as Unum Group is still embroiled in a number of lawsuits filed by plaintiffs who claim they have been unfairly denied disability benefits from the company.
Recently, for instance, Nancy Douglas filed a complaint in US District Court in the southern district of New York, claiming Unum improperly denied her request to obtain long-term disability benefits, according to court documents.
The documents indicate that after careful consideration, the district court ultimately decided to dismiss the complaint against Unum, finding that "there was no abuse of discretion in the denial of benefits."
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Harris, who reportedly suffers from serious brain injuries, claims that Unum asked for an independent medical examination to be conducted to determine the full extend of her injuries, despite the fact that the provider had previously approved her short-term disability claim, according to the Tribune-Democrat.
Unum Group, which is based in Chattanooga, Tennessee, reportedly provides benefits for 37 percent of current Fortune 500 companies and a total of 171,000 businesses in the US and UK, according to its Web site.
The insurance provider was founded in 1848 and merged with Colonial Life and Provident during the 1990s, the Web site said.