Following are the top 10 personal auto insurance companies in the United States ranked by market share. State Farm is the market leader with direct premiums written of over $35.5 billion.

1. State Farm (State Farm Mutual Automobile Insurance Company) is the largest auto insurance company in the US with market share of approximately 19%. One of every five cars is insured by State Farm. With over 65,000 employees, the company has almost 44 million auto insurance policies.  State Farm is also a leading insurer of watercraft in the United States.

2. GEICO (Government Employees Insurance Company) is the second-largest private auto insurer in the United States. With over 13 million auto policies in force, the company insures approximately 23 million vehicles. It holds AA+/Aa1/A++ ratings from Standard & Poor's, Moody's, and A. M. Best and one of the highest rated insurers in the industry. GEICO is a wholly owned subsidiary of Berkshire Hathaway Inc, a leading company in world insurance markets.

3. Allstate Corporation is the third largest auto insurer in terms of market share. It offers private passenger auto and homeowners insurance under the Allstate, Encompass and Esurance brands. The company also offers motorcycle, trailer, motor home, and off-road vehicle insurance policies. As of December 10, 2015, Allstate had a market cap of over $24 billion.

4. Progressive (The Progressive Group of Insurance Companies) is one of the largest car insurance companies in the US. It provides personal and commercial automobile insurance as well as motorcycle, boat, and recreational vehicle insurance (RV Insurance). The company was established in 1937 and is headquartered in Mayfield Village, Ohio.

5. USAA (United Services Automobile Association) is the fifth largest private passenger auto insurer with direct premiums written of over $9.8 billion. The company is headquartered in San Antonio, Texas and employs over 26,000 people. USAA's property and casualty insurance companies are rated A++ (Superior) by A.M. Best, AA+ (Very Strong) by Standard & Poor’s and Aaa (Exceptional) by Moody's.

6. Farmers (Farmers Insurance Group of Companies) is one of the nation's largest insurers of vehicles, homes and small businesses. The Group units include Farmers Insurance, Foremost Insurance, Bristol West Insurance, 21st Century Insurance and Farmers Life.

7. Liberty Mutual offers private passenger automobile, homeowners, and other property-casualty insurance products in the United States. It is the seventh largest personal auto insurance company in the US with market share of over 5%. The company is headquartered in Boston, Massachusetts and operates over 400 offices across the country.

8. Nationwide is one of the leading US property and casualty insurers. It is the country’s eighth largest personal auto insurer. Nationwide provides a full range of insurance products and services including auto, motorcycle, snowmobile, ATV, recreational vehicle, boat, personal watercraft and scooter insurance. The company is headquartered in Columbus, Ohio with regional headquarters in Des Moines, San Antonio, Gainesville, Lynchburg, Raleigh, and Westerville. Nationwide is rated A+ by both A.M. Best and Standard & Poor’s.

9. American Family Mutual is the third-largest mutual insurance company in the US. It specializes in personal and commercial property/casualty and auto insurance. American Family also offers life, health, and home insurance, farm and ranch insurance as well as retirement products. With over 7,500 employees, the company operates through independent agents in Wisconsin, Colorado, Georgia, Idaho, Illinois, Indiana, Iowa, Arizona, Kansas, Minnesota, Missouri, Nebraska, Nevada, North Dakota, Ohio, Oregon, South Dakota, Utah and Washington.

10. Travelers (Travelers Companies Inc) is the 10th largest auto insurance company in the United States based on market share and direct premiums written. It provides a broad array of commercial and personal property and casualty insurance products and services. The company is headquartered in New York with executive offices in Hartford, Connecticut and St. Paul, Minnesota. As of September 30, 2015, Travelers had total assets of $102.1 billion.