Summary of Significant Accounting Policies (Policies)	6 Months Ended
Jun. 30, 2024
Accounting Policies [Abstract]
Nature of Operations and Basis of Presentation	Nature of Operations and Basis of Presentation Arthur J. Gallagher & Co. and its subsidiaries, collectively referred to herein as we, our, us, Gallagher or the company, provide insurance and reinsurance brokerage, consulting and third party claims settlement and administration services to both domestic and international entities. We have three reportable segments: brokerage, risk management and corporate. Our brokers, agents and administrators act as intermediaries between underwriting enterprises and our clients. Our brokerage segment operations provide brokerage and consulting services to entities of all types, including commercial, nonprofit, public sector entities, and, to a lesser extent, individuals, in the areas of insurance and reinsurance placements, risk of loss management, and management of employer sponsored benefit programs. Our risk management segment operations provide contract claim settlement, claim administration, loss control services and risk management consulting for commercial, nonprofit, captive and public sector entities, and various other organizations that choose to self-insure property/casualty coverages or choose to use a third‑party claims management organization rather than the claim services provided by underwriting enterprises. The corporate segment reports the financial information related to our debt and other corporate costs, clean energy investments, external acquisition‑related expenses and the impact of foreign currency translation. We do not assume insurance underwriting risk on a net basis, other than with respect to de minimis amounts necessary to provide minimum or regulatory capital to organize captives, pools, specialized underwriters or risk-retention groups. Rather, capital necessary for covering losses is provided by underwriting enterprises. Interest income, premium finance revenues and other income are primarily generated from our premium financing operations, our invested cash and restricted cash we hold on behalf of our clients, as well as clean energy investments. In addition, our share of the net earnings related to partially owned entities that are accounted for using the equity method is included in other income. We are a global insurance brokerage, risk management and consulting services firm, headquartered in Rolling Meadows, Illinois. We provide these services in approximately 130 countries around the world through our owned operations and a network of correspondent brokers and consultants. We have prepared the accompanying unaudited consolidated financial statements pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures normally included in annual financial statements have been omitted pursuant to such rules and regulations. The unaudited consolidated financial statements included herein are, in the opinion of management, prepared on a basis consistent with our audited consolidated financial statements for the year ended December 31, 2023, and include all normal recurring adjustments necessary for a fair presentation of the information set forth herein. The quarterly results of operations are not necessarily indicative of the results of operations to be reported for subsequent quarters or the full year. These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2023. In the preparation of our unaudited consolidated financial statements as of June 30, 2024, management evaluated all material subsequent events or transactions that occurred after the balance sheet date through the date on which the financial statements were issued, for potential recognition and/or disclosure therein.
Use of Estimates	Use of Estimates The preparation of our unaudited consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. These accounting principles require us to make estimates and assumptions that affect the reported amounts of assets and liabilities and revenues and expenses, and the disclosure of contingent assets and liabilities at the date of our unaudited consolidated financial statements. We periodically evaluate our estimates and assumptions, including those relating to the valuation of goodwill and other intangible assets, right-of-use assets, investments, income taxes, revenue recognition, deferred costs, stock-based compensation, claims handling obligations, retirement plans, litigation and contingencies. We base our estimates on historical experience and various assumptions that we believe to be reasonable based on specific circumstances. Such estimates and assumptions could change in the future as more information becomes known, which could impact the amounts reported and disclosed in the notes herein.

Summary of Significant Accounting Policies (Policies)

6 Months Ended

Jun. 30, 2024

Nature of Operations and Basis of Presentation

Arthur J. Gallagher & Co. and its subsidiaries, collectively referred to herein as we, our, us, Gallagher or the company, provide insurance and reinsurance brokerage, consulting and third party claims settlement and administration services to both domestic and international entities. We have three reportable segments: brokerage, risk management and corporate. Our brokers, agents and administrators act as intermediaries between underwriting enterprises and our clients.

Our brokerage segment operations provide brokerage and consulting services to entities of all types, including commercial, nonprofit, public sector entities, and, to a lesser extent, individuals, in the areas of insurance and reinsurance placements, risk of loss management, and management of employer sponsored benefit programs. Our risk management segment operations provide contract claim settlement, claim administration, loss control services and risk management consulting for commercial, nonprofit, captive and public sector entities, and various other organizations that choose to self-insure property/casualty coverages or choose to use a third‑party claims management organization rather than the claim services provided by underwriting enterprises. The corporate segment reports the financial information related to our debt and other corporate costs, clean energy investments, external acquisition‑related expenses and the impact of foreign currency translation.

We do not assume insurance underwriting risk on a net basis, other than with respect to de minimis amounts necessary to provide minimum or regulatory capital to organize captives, pools, specialized underwriters or risk-retention groups. Rather, capital necessary for covering losses is provided by underwriting enterprises.

Interest income, premium finance revenues and other income are primarily generated from our premium financing operations, our invested cash and restricted cash we hold on behalf of our clients, as well as clean energy investments. In addition, our share of the net earnings related to partially owned entities that are accounted for using the equity method is included in other income.

We are a global insurance brokerage, risk management and consulting services firm, headquartered in Rolling Meadows, Illinois. We provide these services in approximately 130 countries around the world through our owned operations and a network of correspondent brokers and consultants. We have prepared the accompanying unaudited consolidated financial statements pursuant to the rules and regulations of the SEC. Certain information and footnote disclosures normally included in annual financial statements have been omitted pursuant to such rules and regulations. The unaudited consolidated financial statements included herein are, in the opinion of management, prepared on a basis consistent with our audited consolidated financial statements for the year ended December 31, 2023, and include all normal recurring adjustments necessary for a fair presentation of the information set forth herein. The quarterly results of operations are not necessarily indicative of the results of operations to be reported for subsequent quarters or the full year. These unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto included in our Annual Report on Form 10-K for the year ended December 31, 2023. In the preparation of our unaudited consolidated financial statements as of June 30, 2024, management evaluated all material subsequent events or transactions that occurred after the balance sheet date through the date on which the financial statements were issued, for potential recognition and/or disclosure therein.

Use of Estimates

The preparation of our unaudited consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. These accounting principles require us to make estimates and assumptions that affect the reported amounts of assets and liabilities and revenues and expenses, and the disclosure of contingent assets and liabilities at the date of our unaudited consolidated financial statements. We periodically evaluate our estimates and assumptions, including those relating to the valuation of goodwill and other intangible assets, right-of-use assets, investments, income taxes, revenue recognition, deferred costs, stock-based compensation, claims handling obligations, retirement plans, litigation and contingencies. We base our estimates on historical experience and various assumptions that we believe to be reasonable based on specific circumstances. Such estimates and assumptions could change in the future as more information becomes known, which could impact the amounts reported and disclosed in the notes herein.