Frustrated by their annual wrangling over health care reimbursement rates, care provider Fairview Health Systems and insurer Medica decided to try something more collaborative. They didn’t know it two years ago, but they were forming one of the country’s first “accountable care organizations” or ACOs. The model they set up reduces costs for both organizations and it has national influence. But what is an ACO? And will employers get in on the savings, too?