Symetra Financial Corp., a life insurer that counts Berkshire Hathaway Inc. among its largest shareholders, joined Apollo Global Management LLC to bid for Aviva PLC’s U.S. unit, two people with knowledge of the matter told Bloomberg.

Apollo and Symetra are in advanced talks to buy the unit and may reach a deal within days, said the people, who spoke on condition of anonymity because the talks are private. The talks may fail to lead to an agreement, and the bidding group could change, the people said.

Guggenheim Partners LLC, which had earlier paired with Apollo to pursue the bid for the Aviva assets, is no longer part of the group, Bloomberg said. A sale would occur at a “substantial discount” to the unit’s book value, excluding debt, of $3.88 billion, Aviva Chief Financial Officer Pat Regan said last month.

Aviva acquired most of its U.S. operations through the $2.9 billion purchase of Des Moines-based AmerUs Group Co. in 2006. Read more.