NSM Insurance Group, a specialty insurance programs administrator, has announced that it has entered into an agreement for White Mountains Insurance Group to acquire a majority stake in the firm.

“White Mountains brings NSM strong financial backing and a long-term view of the business, which will enable us to continue our organic and acquisition growth strategies both in the US and UK,” said NSM CEO Geof McKernan.

NSM, based in Pennslyvania, deals with around 100 insurance carriers and around 6,000 brokerages placing in excess of $500 million through annual premiums. It has niches includes non-profits, collector cars, pet insurance, specialty real estate and sports and wellness centers.

“We are pleased to make this investment in NSM alongside the management team,” said Manning Rountree, CEO of White Mountains. “We look forward to working with Geof and his team, and to providing them with support and resources to continue to grow the business organically and through acquisitions.”

Over the past 12 years, NSM has completed more than 12 transactions, launched eight new program ventures, and is actively seeking insurance business in both the US and the UK.

The transaction with White Mountains is expected to close by the end of the second quarter, subject to regulatory approval and other conditions.