EXOR SpA offered $6.4 billion for PartnerRe Ltd., seeking to break up the reinsurer's planned merger with AXIS Capital Holdings Ltd. EXOR, controlled by the Agnelli family, is offering $130 a share in cash, the company said in a statement Tuesday, or 16 percent above the implied value per share of Bermuda-based PartnerRe under the terms of the AXIS agreement. An AXIS-PartnerRe combination would create the world's fifth-biggest property and casualty reinsurer, and was announced in January , the same month that XL Group plc agreed to buy Catlin Group Ltd. for about $4 billion.