If you’re looking for a thorough description of all things ACO, check out the recent publication by Robert Berenson and Rachel Burton (ungated). Included in it is the following about private insurer ACOs. I was not aware of all of this.
At least eight private health insurance plans have entered into ACO contracts with providers using a shared risk payment model, making providers eligible for both bonuses and financial penalties. Many more (27, by one count) have entered into shared savings contracts, which make providers eligible for bonuses, but do not put them at financial risk if they exceed spending targets. [...]
The private ACO contracts that have been identified so far use shared savings, shared risk, or partial capitation; none have moved all the way to full capitation yet. [...]
The private ACO contracts using shared risk include several Blue Cross Blue Shield plans in different states (e.g., Illinois, Massachusetts, New Jersey, North Carolina), Aetna, and Anthem/WellPoint.