Banks, New York Regulator Meet on Bond Insurer Rescue New capital may help preserve the top credit ratings for the bond guarantors such as MBIA Inc., the industry's largest, and halt any erosion of investor confidence in the $2 trillion of assets they guarantee. Ambac Financial Group Inc., MBIA's biggest rival, lost its AAA grade from Fitch Ratings this month on concern about rising defaults tied to subprime mortgages.