The Collaborative International Dictionary of English v.0.48:

Insurer \In*sur"er\, n.
   One who, or that which, insures; the person or company that
   contracts to indemnify losses for a premium; an underwriter.
   Insurgence

WordNet (r) 3.0 (2006):

insurer
    n 1: a financial institution that sells insurance [syn:
         insurance company, insurance firm, insurer,
         insurance underwriter, underwriter]