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Restricted Stock and Performance Share Awards - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2026 | Sep. 30, 2025 | Mar. 31, 2025
Minimum | Employee Employed By Gallagher |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |
Vesting period, years |  | 2 years |
Maximum | Employee Employed By Gallagher |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |
Vesting period, years |  | 5 years |
Restricted Stock Units (RSUs) | Employee Employed By AssuredPartners |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |
Shares granted in the period (in shares) |  | 341,700 |
Aggregate grant-date fair value |  | $ 100 |
Restricted Stock Units (RSUs) | Employee Employed By Gallagher |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |
Shares granted in the period (in shares) |  | 708,000 |
Aggregate grant-date fair value |  | $ 215 |
Long Term Incentive Plan |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |
Shares available for grant (in shares) | 1,500,000 |  |
Long Term Incentive Plan | Restricted Stock Restricted Stock Units And Performance Unit Awards |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |
Shares authorized (in shares) | 4,000,000.0 |  |
Long Term Incentive Plan | Restricted Stock Units (RSUs) |  |  |
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] |  |  |
Shares granted in the period (in shares) | 500,000 |  | 300,000
Aggregate grant-date fair value | $ 115 |  | $ 94

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- Definition Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items] + References No definition available. + Details
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- Definition Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Fair Value + References No definition available. + Details
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- Definition Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Number of shares authorized for issuance under share-based payment arrangement. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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