Marsh & McLennan Companies, Inc.

Marsh & McLennan sits at the centre of a three-part chain that moves a multinational corporation's risk from assessment through to reinsurance: Mercer's actuaries model the exposures using proprietary software already embedded in clients' regulatory filings, Marsh then places the resulting insurance across 130 jurisdictions where it holds the local intermediary licences required by law, and Guy Carpenter routes the reinsurance into Lloyd's of London syndicates under a broker accreditation built from decades of trading history that no new entrant can simply purchase. Because each stage depends on the previous one — Lloyd's syndicates set quotation terms partly on the basis of the cession flow Guy Carpenter has historically delivered, and that flow only exists because Marsh's primary placements are generating premium — a competitor would need to replicate all three accreditation histories at once to offer a client the same coordinated coverage. The part of the business hardest to replace is not the licences, which can be acquired by buying local brokerages, but the senior brokers whose personal relationships with carriers and syndicate underwriters are what those licences are actually worth in practice, and those relationships accumulate only through years of supervised placements inside the firm. The whole chain snaps if Guy Carpenter's Lloyd's accreditation is revoked, because without it the reinsurance step disappears and the single-adviser relationship that justifies the entire arrangement loses its reason to exist.