Here is a partial definition in French, followed by an English equivalent from another source:-

Les assureurs privés sont tous contraints par les traités de réassurance. Généralement, dans les traités de réassurance, l'assureur de première ligne émet la police et réassure 75 ou 80 p. 100 du risque auprès d'autres assureurs.
http://www.parl.gc.ca/committees/fore/evidence/72_95-10-24/f...

underlier: assureur de base, assureur de première ligne (LGA)
http://www.actuaries.ca/publications/lexicon/lextu.html

My level of confidence here is zero, because I have never heard that specific term ("underlier") before. Maybe someone else can provide more information.

Good luck in your further search for an appropriate translation.

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Note added at 2002-08-06 05:07:11 (GMT)
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More info. It appears that an \"underlier\" is also a kind of policy and a kind of investment. So a more precise term would be preferrable.

Also it appears that one can speak of \"primary\" and \"derivative instruments\" the former being synonymous with \"underlier\" -- but whether that usage can be extended to Insurers, I do not know.

See:-

Underlier: -- A foreign, admitted policy written to comply with the countries laws, which is then supplemented by a DIC written by a nonadmitted carrier.
http://www.iiat.org/work/cgl/glossaryforeign.htm

underlier A primary instrument or variable upon which the value of a derivative instrument depends.
http://www.contingencyanalysis.com/glossary.htm

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Note added at 2002-08-06 05:11:31 (GMT)
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Maybe it\'s \"primary\" and \"secondary insurer\". It could be as simple as that. See (inter alia):-

Noridian Medicare: Beneficiary: Coverage: Medicare as Secondary ... A secondary payer covers medical expenses only after a primary insurer has paid a claim. For example, if a claim for $100 were filed ...
http://www.noridianmedicare.com/bene/coverage/secondary_paye...

Summary of HB4975 ... insurance coverage. Assigns the primary and secondary insurer based on the amount of insurance the permitted user has. Provides that ...
http://www.legis.state.il.us/scripts/imstran.exe?LIBSINCWHB4...

NPRM: Standards for Electronic Transactions ... the claim to the primary insurer, receives payment, and resubmits the claim (with the remittance advice from the primary insurer) to the secondary insurer. ...
http://aspe.hhs.gov/admnsimp/nprm/tx08.htm

Claims Filing Requirements The claims filing requirements for ... (PDF) ... A beneficiary who submits a claim to the primary insurer might personally submit a secondary claim to Medicare on a HCFA-1500 claim form, or might forward the ...
http://www.nationwide-medicare.com/medpol/Volume 1/guide/Cha...

HIPAA Requirements - Coordination of Benefits ... receives partial payment, and then the provider resubmits the same claim (with
the remittance advice from the primary insurer) to the secondary insurer. ...
http://www.dhmh.state.md.us/HIPAA/html/cobenefdef.htm

Etceter, etcetera -- very many references.

Jacques Lesobre's insurance dictionary gives the following:

police de deuxième (etc) ligne: first (etc) excess policy
police de première ligne: underlying= master policy

premère ligne: working=first= primary layer

Hope this helps
Mary

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Note added at 2002-08-06 06:53:15 (GMT)
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May I just say that I am not an insurance expert, only happen to have an insurance dictionary so thought I might be able to help.

I also found "underlier" in the context of Canadian actuaries, and looks correct to me (the "basic" insurer).
But it's difficult to combine "première" with "seconde ligne" (unless you're in the trade and know the specific word (?), of course, which I don't).

BTW, the financial "underlier" you mention is the "sous-jacent".

So I suggest "first-tier" and "second-tier insurer".
You'll find 3 Google references about "second-tier insurer" and a few more about "first-tier insurer".
None is a definition.
"first-tier" and "second-tier" are absent from Eurodicautom.

HTH

?

Jerrie is absolutely right in her answer.The master concept here is the reinsurance that all insurance Companies do,redistributing their risks with other insurers,underwriting them.
This only pretends to sustent her answer.