AM Best has assigned an issuer credit rating of 'a-' and affirmed the financial strength rating of A- (excellent) of Bermuda-based Bison Insurance Company Limited.

The outlook for the financial strength rating is stable, and the outlook assigned to the issuer credit rating is stable.

These ratings reflect Bison's historically adequate capitalisation, generally favorable operating performance, conservative reserve levels and effective management of its catastrophe exposures. The ratings also recognise Bison's history of maintaining sufficient capital and financial resources to support its ongoing obligations.

Partially offsetting these positive rating factors is the company's volatile underwriting results due to the historically low loss frequency, high loss severity nature of its risk profile, coupled with its high limits and high net retentions relative to surplus. Also, the continually changing risk profile of Bison's primary insurer causes large profile changes that affect the captive.

Bison is the single parent captive of Duke Energy and its subsidiaries.

Meanwhile, the ratings agency also has assigned an issuer credit rating of 'a' and affirmed the financial strength rating of A (excellent) of fellow Bermuda company PMG Assurance Ltd.

These ratings reflect PMG's good capitalisation, historically strong operating performance and strategic position as the captive insurance company for Sony Group (Sony).

These strengths are partially offset by the company's exposure to potentially large natural catastrophe losses and its limited business profile as a capacity provider in the open reinsurance market.

PMG's role is to meet certain global insurance requirements of Sony, as well as to provide reinsurance capacity to a number of third party insurers and reinsurers.