 Insurance companies may be classified into two groups. Life insurance companies, which sell life insurance, annuities and pensions products. Non-life or property slash casualty insurance companies, which sell other types of insurance. General insurance companies can be further divided into these subcategories. Standard lines access lines in most countries. Life and non-life insurers are subject to different regulatory regimes and different tax accounting rules. The main reason for the distinction between the two types of company is that life, annuity, and pension business is very long-term in nature. Coverage for life insurance or pension can cover risks over many decades. By contrast, non-life insurance cover usually covers a shorter period, such as one year. Insurance companies are generally classified as either mutual or proprietary companies. Thirty-six mutual companies are owned by the policy holders, while shareholders who may or may not own policies own proprietary insurance companies. The mutualization of mutual insurers to form stock companies, as well as the formation of a hybrid known as a mutual holding company, became common in some countries, such as the United States, in the late 20th century. However, not all states permit mutual holding companies. Other possible forms for an insurance company include reciprocals, in which policy holders reciprocate in sharing risks, and voids organizations. Insurance companies are rated by various agencies such as AMBEST. The ratings include the company's financial strength, which measures its ability to pay claims. It also rates financial instruments issued by the insurance company, such as bonds, notes, and securitization products. Re-insurance companies are insurance companies that sell policies to other insurance companies, allowing them to reduce their risks and protect themselves from very large losses. The re-insurance market is dominated by a few very large companies, with huge reserves. The re-insurer may also be a direct writer of insurance risks as well. Captive insurance companies may be defined as limited-purpose insurance companies established with the specific objective of financing risks emanating from their parent group or groups. This definition can sometimes be extended to include some of the risks of the parent company's customers. In short, it is an in-house self-insurance vehicle. Captives may take the form of a pure entity which is a 100% subsidiary of the self-insured parent company of a mutual captive which ensures the collective risks of members of an industry and of an association captive which self-insures individual risks of the members of a professional, commercial or industrial association. Captives represent commercial, economic and tax advantages to their sponsors because of the reductions in costs they help create and for the ease of insurance risk management and the flexibility for cash flows they generate. Additionally, they may provide coverage of risks which is neither available nor offered in the traditional insurance market at reasonable prices.