The Collaborative International Dictionary of English v.0.48:

Assurer \As*sur"er\, n.
   1. One who assures. Specifically: One who insures against
      loss; an insurer or underwriter.
      [1913 Webster]

   2. One who takes out a life assurance policy.
      [1913 Webster]

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

ASSURER. One who insures another against certain perils and dangers. The
same as underwriter. (q.v.) Vide Insurer.