one year or less when purchased. In addition, we hold other investments, including direct private equity investments, catastrophe bonds, fund investments and term loans, which offer the potential for higher returns but with relatively higher levels of risk. Our investment portfolio takes into account the duration of our liabilities and the level of strategic asset risk we wish to assume over the medium- to long-term. We may from time to time re-evaluate our investment guidelines and explore investment allocations to other asset classes that either increase or decrease our overall asset risk. Our investments are subject to market-wide risks and fluctuations, as well as to risks inherent in particular securities. For additional information regarding our investment portfolio, refer to “Part II, Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations—Financial Condition, Liquidity and Capital Resources—Investments” and “Note 5. Investments” in our “Notes to the Consolidated Financial Statements.” MARKETING We believe that our modeling and technical expertise, the risk management products we provide to our customers, and our reputation for paying claims promptly has enabled us to become a provider of first choice in many lines of business to our customers worldwide. We market our products primarily through reinsurance brokers and we focus our marketing efforts on targeted brokers and partners. We believe that our existing portfolio of business is a valuable asset and, therefore, we attempt to continually strengthen relationships with our existing brokers and customers. We believe that by maintaining close relationships with brokers, we are able to obtain access to a broad range of potential reinsureds. We target prospects that are capable of supplying detailed and accurate underwriting data and that potentially add further diversification to our book of business. We believe that primary insurers’ and brokers’ willingness to use a particular reinsurer is based not just on pricing, but also on the financial security of the reinsurer, its claim paying ability ratings and demonstrated willingness to promptly pay valid claims, the quality of a reinsurer’s service, the reinsurer’s willingness and ability to design customized programs, its long-term stability and its commitment to provide stable reinsurance capacity across market cycles. We believe we have established a reputation with our brokers and customers for prompt response on underwriting submissions, for fast payments on valid claims and for providing creative solutions to our customers’ needs. Our portfolio of business continues to be characterized by relatively large transactions with ceding companies with whom we do business, although no current relationship exceeds 10% of our gross premiums written. Our brokers assess client needs and also perform data collection, contract preparation and other administrative tasks, enabling us to market our products cost effectively. Our distribution is reliant on a small number of broker relationships, which has continued to decrease in recent years as a result of consolidation in the broker sector. We expect this concentration to continue. In 2021, three brokerage firms accounted for 78.0% of our gross premiums written. The following table shows the percentage of our Property and Casualty and Specialty segments’ gross premiums written generated through subsidiaries and affiliates of our largest brokers: Year ended December 31, 2021 Property Casualty and Specialty Total Aon plc 41.8 % 29.7 % 35.8 % Marsh & McLennan Companies, Inc. 25.6 % 34.5 % 30.0 % Arthur J. Gallagher (1) 5.8 % 18.7 % 12.2 % Total of largest brokers 73.2 % 82.9 % 78.0 % All others 26.8 % 17.1 % 22.0 % Total 100.0 % 100.0 % 100.0 % (1) Includes the percentage of gross premiums written for the year ended December 31, 2021 which were generated through Willis Re, a subsidiary of Willis Towers Watson Public Limited Company, which was acquired by Arthur J. Gallagher on December 1, 2021. 17