White Mountains Insurance Group

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White Mountains Insurance Group (NYSEWTM) is a holding company with business interests in property and casualty insurance, and reinsurance. The group owns reinsurer Sirius and a 75% stake in specialty insurance carrier OneBeacon.[1][2] It is based in Hanover, New Hampshire.

It was founded by Jack Byrne who in 1985 was invited to run the troubled Fireman's Fund, then a subsidiary of American Express. Fireman's had incurred $356 million in pretax losses in 1983 and 1984. Byrne greatly improved Fireman's financial performance and initiated a public offering of some of Fireman's shares in 1985. The company was sold to Allianz AG in 1991. Byrne, meanwhile, retained the Fireman's holding company, which he later renamed White Mountains.[3][4]


  1. ^ White Mountains to Acquire Sirius Insurance
  2. ^ OneBeacon IPO raises $600M on NYSE
  3. ^ John Gorham, "See Jack Run" Forbes.com, July 24, 2000
  4. ^ White Mountains Insurance Group Ltd., Company History

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