SAN FRANCISCO (AP) Health insurer Cigna has rejected a $47 billion offer to be acquired by its larger rival, Anthem.

Cigna's board says the proposal is inadequate and not in the interests of its shareholders, according to a letter released Sunday by Cigna, which is based in Bloomfield, Connecticut. The letter outlined several objections to the deal.

Anthem has had talks with Cigna about a deal since last year. After failing to reach agreement on earlier proposals, Indianapolis-based Anthem went public with its latest offer on Saturday.

Anthem's latest offer was a cash and stock deal that amounted to $184 for each Cigna share, or about 18 percent more than Cigna shares closed on Friday.