For 2005, the five largest auto insurance companies, measured by size of direct premiums written, were: State Farm Mutual Group; Allstate Insurance Co. Group; Progressive Casualty Group; National Indemnity Co. Group (Berkshire Hathaway); and Farmers Insurance Group.
(Insurance Information Institute: Facts and Statistics: Auto Insurance -- scroll halfway down the page to "Leading Writers of Private Passenger Auto Insurance by Direct Premiums Written, 2005")