Depending on your current job title and which division you're in, there are promotional opportunities for high performers. The overall caliber of people working there is very high - I've had three previous employers and there is no comparison.
On the down side, Anthem suffers from the same dilbert-esque environment you find in big, publicly-held companies. The relentless pressure of performing financially overshadows the ability to invest to the degree needed and the employees have paid the price in terms of benefits and job security. I've been at the company 17 years and I'm on my 15th boss. Job cuts are frequent and reorganization never ends. In some types of roles, you can get bounced from department to department, essentially learning a new job every year. Sometimes you get dropped into departments that don't necessarily want you. By the time you get acclimated to a new role, they bounce you somewhere else. It makes it hard to get real traction in your career.