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]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "insurer":
   approver, bailsman, bondsman, certifier, confirmer, cosignatory,
   cosigner, endorser, godfather, godparent, guarantee, guarantor,
   guaranty, mainpernor, mortgagor, notary, notary public, party,
   ratifier, signatory, sponsor, subscriber, surety, underwriter,
   upholder, warrantor



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSURER, contracts. One who has obliged himself to insure the safety of
another's property, in consideration of a premium paid, or secured to be
paid, to him. It is his duty to pay any loss which has arisen on the
property insured. Vide Marsh. Ins. Index,.h. t.; Park. Ins. Index, h. t.
Phill. Ins. h. t.; Wesk. Ins. h. t.; Pardess. Index, art. Assureur.