What is treaty reinsurance and how does it work for commercial property carriers?

Treaty reinsurance is one of the foundations of how P&C carriers manage capital and CAT exposure. Every modern carrier writing meaningful commercial property runs at least one treaty — and the treaty's shape constrains every individual underwriting decision.

Two families of treaty

Proportional treaties cede a fixed share of every risk:

• Quota Share (QS). The reinsurer takes X% of every risk and pays X% of every loss. Useful for new lines, capital relief, and carriers expanding into new geographies. Cession typically 20–60%.
• Surplus Share (SS). The reinsurer takes the slice of each risk that exceeds the primary's retention. Carrier sets its "line" (e.g. $5M); anything above goes to surplus share up to the treaty limit. Useful for managing very large individual risks without ceding the small stuff.

Non-proportional (XL) treaties cede losses above an attachment point:

• Per-Risk XL. Reinsurer pays losses on a single risk above the attachment, up to the layer limit. Protects against unusual large-single-risk losses.
• CAT XL. Reinsurer pays accumulated losses from a single CAT event (named storm, earthquake, wildfire) above the attachment. The carrier's primary protection against tail catastrophe events.
• Aggregate XL. Reinsurer pays accumulated losses across all events above an annual aggregate retention. Sits behind CAT XL in many programmes.

How treaty terms drive every underwriting decision

When a carrier binds a $50M TSI commercial property risk in Miami: - The QS treaty takes its slice (e.g. 30% of premium → 30% of any loss) - The SS treaty might take another slice above the carrier's $5M per-risk line - The CAT XL sits behind, kicking in only if a named storm produces a $50M+ accumulated event - The carrier's net retained exposure (after all cessions) is the number that hits its capital

Every bind has to fit inside: - Treaty utilisation — total cession to each treaty layer so far in the treaty year, against the layer limit - Treaty appetite — some treaties exclude certain occupancies, states, or perils. A binding outside that appetite is unrecoverable. - Treaty pricing — the cession premium back to the reinsurer is part of the deal economics. Below-band pricing on the primary side erodes the reinsurer's position too.

Reinsurance market dynamics (2025–2026)

• 2022–2023 saw the hardest reinsurance market in two decades. CAT XL pricing rose 35–50% per renewal. Attachment points moved up, meaning carriers retained more.
• 2024–2025 stabilised, with rate increases in the 5–10% range and some softening in non-CAT lines.
• Florida coastal property and California wildfire continue to be the hardest treaty placements.

How AI underwriting platforms handle treaty

Modern multi-agent AI platforms run a treaty agent in the pipeline: - Loads the treaty programme summary (layers, attachments, limits, utilisations, appetites) - Checks each bind against treaty utilisation in real time - Returns a "fits / doesn't fit / refer" verdict to the memo synthesis - Generates a treaty cession draft as part of the bound-risk record

This replaces the manual quarterly treaty reconciliation that most carriers run today — at the cost of one well-built agent and a clean treaty data feed.

Reference sources

• Reinsurance Association of America (RAA)
• Lloyd's — How Lloyd's reinsurance works