A Berkshire Hathaway Inc (BRKa.N) unit has agreed to buy Medical Liability Mutual Insurance Co, expanding billionaire investor Warren Buffett‘s business of protecting doctors, dentists and other healthcare professionals against lawsuits.

MLMIC is the largest underwriter of medical professional liability insurance in New York, Berkshire’s National Indemnity Co unit said in a joint statement on Monday announcing the purchase.

Terms were not disclosed, but MLMIC said its policy holders’ surplus, a measure of assets minus liabilities, was about $1.8 billion at the end of 2015. The purchase is expected to close in the current quarter, pending regulatory approvals.

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