Marsh & McLennan Companies, Inc. (MRSH)

Gallagher competes head-on for mid-market and large commercial insurance placements in the United States and, through Gallagher Re, for the reinsurance treaties Guy Carpenter places.

Brown & Brown fights Marsh McLennan Agency for the same American mid-market and small-business clients, buying up regional agencies in the same towns to win their commission income.

Aon is the closest match to Marsh McLennan across every line the group operates — commercial insurance broking for large corporations, reinsurance broking through Aon Reinsurance Solutions against Guy Carpenter, and health and retirement consulting against Mercer.

WTW bids for the same multinational corporate accounts in property and casualty broking and, with its benefits and actuarial practice, for the same pension and health advisory mandates that Mercer pursues.

Lockton is the largest privately held broker in the world and takes corporate risk and employee-benefit accounts directly from Marsh, pitching its independence to clients that dislike listed brokers.

Howden has grown into a global broker across Europe, Asia and Latin America and, with Howden Re, bids against Guy Carpenter for reinsurance and specialty placements.