(Bloomberg) ? Bermuda-incorporated White Mountains Insurance Group Ltd.?s Folksamerica Holding agreed to buy a workers? compensation insurance unit of Sierra Health Services Inc., a managed-care company, for $79.5 million.

Sierra Health, based in Las Vegas, will get $15.5 million for California Indemnity Insurance Co. when the sale closes, probably by April, and as much as $64 million in January 2010, the company said in a statement.

Michael Tyburski, a spokesman for Folksamerica in New York, didn?t immediately return a call to his office. The workers? compensation business isn?t part of Sierra Health?s core business and the company has been seeking a buyer since January, spokesman Peter O?Neill said. Sierra expects a related fourth-quarter expense of $15 million to $20 million, the difference between the unit?s book value and the sale price.

?It?s a very tough business to be in,? O?Neill said.

?There?s a lot of uncertainty in it that makes it difficult to adequately predict claims and expenses.?

The workers? compensation unit operates in nine states in the US West and Midwest.

The transaction doesn?t include Sierra Health?s Nevada-based workers? compensation business, which provides coverage for a few companies, O?Neill said.

Shares of White Mountains, which is incorporated in Bermuda and has offices in Hanover, New Hampshire, rose $1.50 to $425 in New York Stock Exchange composite trading. They?ve risen 32 percent this year.

Sierra Health rose 9 cents to $27.58. It?s more than doubled this year.