Reinsurance is critical to the insurance process; it brings capacity, stability, and financial strength to insurers. The purpose of reinsurance is to spread large risks and catastrophes over as large a base as possible. It is the assumption by one insurance company (the reinsurer) of all or part of a risk undertaken by another insurance company (the cedent). It enables an insured with a sizable risk exposure to deal with and receive coverage from one insurer, rather than dealing with a number of insurers. The portion of the risk that exceeds the primary insurer's retention level is layed-off (ceded) to a reinsurer. The reinsurer can further reinsure a part of the risk assumed; this is called retroceding. If the reinsurer agrees to share losses arising from only one risk, the agreement is known as facultative reinsurance; if the reinsurer agrees to share losses arising from more than one risk, usually a whole line or book of business, the agreement is known as treaty reinsurance. Western Europe is the largest provider of worldwide reinsurance. The Caribbean, including Bermuda, is the largest foreign supplier of reinsurance to the United States. The financial strength of the reinsurer is most important, since the direct writer is always primarily responsible for payment of losses.