The US reinsurance division of Bermuda-based White Mountains Insurance Group Ltd has had its financial strength rating upgraded to A from A- by AM Best Co.The ratings change for Sirius America Insurance Company comes after White Mountains reorganised its reinsurance operations last month, bringing together all of its reinsurance operations under Sirius International Group.The rating agency noted that regulatory approvals have been received for the changes.Best said that Sirius America’s ratings reflected the strategic importance of the US operating company to Stockholm-based parent Sirius International, its excellent level of risk-adjusted capitalisation and enhanced market profile and risk management strategy as a wholly owned subsidiary of Sirius International.“The reorganisation is expected to improve capital and tax efficiency across the group, as well as enhance the diversification of the overall Sirius’ portfolio,” Best said in its commentary. “Sirius’ regulatory capital is expected to increase as a result of the reorganisation.“As a result of these global strategies, Sirius America has improved its overall performance for the first half of 2011; however, a positive long-term trend has yet to be established.”