Pros
Stability, stability...stability. Because people tend to spend the majority of their careers here, there is a tremendous amount of resident knowledge specific to commercial property insurance that has and continues to serve the company and its policyholders very well. Also, the company is a mutual insurer, so senior management ultimately runs the company (not Wall Street).
Cons
If you are looking for career advancement over a reasonable timeframe, be prepared to watch ice bergs melt. Because people tend to spend the majority of their careers here, professional advancement tends to be stifled in favor of job stability. This means political positioning is a significant factor in determining who gets what when opportunities open up. Also, there is very much of a "if it isn't broken, don't fix it" mentality on the part of management. Since the company has been in business for over 175 years and its business model is all about avoiding risk (for itself and policyholders), there is usually a tremendous amount of resistance towards new ideas, even when these make sense. This is particularly true for newer hires that come in with a head of steam wanting to make a positive difference - the "old-timers" tend to look down on these folks as naive renegades. Accordingly, there are few middle managers that have the courage to stick their necks out for employees that may draw attention to themselves.