MBIA Inc. posted the largest quarterly loss in its history after marking down the value of residential and commercial mortgages and the collateralized debt obligations the bond insurer guarantees. The world's largest bond insurer said it is selling shares to raise capital. MBIA wants to avoid becoming the fourth bond insurer to be stripped of its AAA credit rating. Without the AAA stamp, MBIA could not offer new securities its top rating, crippling its business and effecting the ratings of $652 billion of debt it insures. Banks could be forced to write down $70 billion because of the downgrades at bond insurers, Oppenheimer & Co. analyst Meredith Whitney said.