In the United States, the two largest life insurance companies, in terms of assets as of 2015, are Metlife and Prudential Financial. The largest domestic commercial and personal lines insurer is State Farm Mutual Insurance. United Healthcare is the biggest health insurance and managed health care company.

Warren Buffett's Berkshire Hathaway is actually the biggest company that provides some insurance services even though it's not primarily an insurer. By end of 2013, Berkshire possessed a market capitalization of more than three times that of United Healthcare ($308 billion to $92 billion), the largest company where the primary product is insurance.

In practice, the largest insurer in the world is the Federal Reserve, which backs both the Federal Deposit Insurance Corporation (FDIC) and much of the International Monetary Fund (IMF). However, the Fed is not considered part of the insurance sector.

Subsectors in the Insurance Sector

There are four general sub-categories in the greater insurance sector: life insurance, property and casualty (personal lines), accident and health insurance, and miscellaneous.

Personal lines are insurance policies written as protections against specific unforeseen events. State Farm, as of 2015, sells the most property and casualty insurance, although its market cap is smaller than that of Berkshire Hathaway or American International Group (AIG).

Accident and health insurance policies deal with unspecific risks and accidents. The bulk of this subsector is focused on reducing the personal cost of health care. United Healthcare, part of UnitedHealth Group, is the largest player in this area as of 2015.

MetLife and Prudential Financial are the biggest life insurance providers by a wide margin. When measured by total assets related to insurance products and the provision of insurance services, these are the two largest insurance companies in the world as of 2015.

Other insurers, such as Ambac Financial Group or MBIA, insure financial instruments and other accounts against counterparty risk. These are sometimes called "assurance companies" rather than insurance companies.

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