Proportional

Under proportional reinsurance, one or more reinsurers take a stated percentage share of each policy that an insurer produces ("writes"). This means that the reinsurer will receive that stated percentage of the premiums and will pay the same percentage of claims. In addition, the reinsurer will allow a "ceding commission" to the insurer to cover the costs incurred by the insurer (marketing, underwriting, claims etc.).

The arrangement may be "quota share" or "surplus reinsurance" (also known as surplus of line or variable quota share treaty) or a combination of the two. Under a quota share arrangement, a fixed percentage (say 75%) of each insurance policy is reinsured. Under a surplus share arrangement, the ceding company decides on a "retention limit" - say $100,000. The ceding company retains the full amount of each risk, with a maximum of $100,000 per policy or per risk, and the balance of the risk is reinsured.

The ceding company may seek a quota share arrangement for several reasons. First, they may not have sufficient capital to prudently retain all of the business that it can sell. For example, it may only be able to offer a total of $100 million in coverage, but by reinsuring 75% of it, it can sell four times as much.

The ceding company may seek surplus reinsurance simply to limit the losses it might incur from a small number of large claims as a result of random fluctuations in experience. In a 9 line surplus treaty the reinsurer would then accept up to $900,000 (9 lines). So if the insurance company issues a policy for $100,000, they would keep all of the premiums and losses from that policy. If they issue a $200,000 policy, they would give (cede) half of the premiums and losses to the reinsurer (1 line each). The maximum automatic underwriting capacity of the cedant would be $1,000,000 in this example. (Any policy larger than this would require facultative reinsurance.)

Non-proportional

Under non-proportional reinsurance the reinsurer only pays out if the total claims suffered by the insurer in a given period exceed a stated amount, which is called the "retention" or "priority". For instance the insurer may be prepared to accept a total loss up to $1 million, and purchases a layer of reinsurance of $4 million in excess of this $1 million. If a loss of $3 million were then to occur, the insurer would bear $1 million of the loss and would recover $2 million from its reinsurer. In this example, the insured also retains any excess of loss over $5 million unless it has purchased a further excess layer of reinsurance.