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Restricted Stock, Performance Share and Cash Awards (Detail) $ in Millions				3 Months Ended				9 Months Ended
	Mar. 01, 2025 USD ($) shares	Mar. 01, 2024 USD ($) shares	Mar. 15, 2023	Sep. 30, 2025 USD ($) shares	Mar. 31, 2025 USD ($) shares	Sep. 30, 2024 USD ($) shares	Mar. 31, 2024 USD ($) shares	Sep. 30, 2025 USD ($) TIME shares	Sep. 30, 2024 USD ($) shares	Mar. 15, 2022 USD ($) shares
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Vesting period, years	3 years
Cash-based compensation awards, expenses				$ 0.0				$ 0.0
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)
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Restricted stock or unit expense				$ 23.0		$ 13.2		52.3	$ 40.2
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) | shares				341,700
Fair value of grants in period				$ 100.0
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) | shares					708,000
Fair value of grants in period					$ 215.2
Unvested Restricted Stock
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Total intrinsic value				779.9		549.7		779.9	549.7
Equity awards with an aggregate fair value								212.7	93.3
Performance Shares
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Restricted stock or unit expense				6.6		5.7		19.2	16.3
Total intrinsic value				$ 104.9		92.2		104.9	92.2
Equity awards with an aggregate fair value								$ 35.5	$ 31.6
Performance share awards approved, Fair value	$ 21.8	$ 14.2
Vesting period, years	3 years	3 years	3 years
Cash Awards
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Shares granted in the period (in shares) | shares								0	0
Cash-based compensation awards, expenses						$ 3.3			$ 10.0
Cash Awards | Officer and Key Employees
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Vesting period, years								3 years
Performance awards period, years								1 year
Ultimate award value, multiples of original value of the units, minimum | TIME								0.5
Ultimate award value, multiples of original value of the units, maximum | TIME								1.5
Provisional compensation cash awards approved for future grant by compensation committee, value										$ 19.9
Provisional compensation cash award approved for future grant by compensation committee(in units) | shares										125,000
2022 Provisional Cash Awards
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Provisional compensation cash award approved for future grant by compensation committee(in units) | shares				111,000				111,000
Cash award with aggregate fair value vested and distributed to participants								$ 26.5
2021 Provisional Cash Awards
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Provisional compensation cash award approved for future grant by compensation committee(in units) | shares						129,000			129,000
Cash award with aggregate fair value vested and distributed to participants									$ 25.4
Long Term Incentive Plan | Executive Officer | Minimum
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Minimum age of employee with not subject to award forfeiture on condition compliance				62 years
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) | shares				4,000,000.0				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) | shares					297,000		345,000
Fair value of grants in period					$ 94.3		$ 83.9
Long Term Incentive Plan | Performance Shares
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Shares authorized (in shares) | shares	68,000	58,000
Share-based compensation arrangement, conversion ratio to common stock	1	1
Long Term Incentive Plan | Performance Shares | Executive Officer | Minimum
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Minimum age of employee with not subject to award forfeiture on condition compliance		62 years	55 years
Period of service from grant date stock options awarded not subject to forfeiture			2 years
Long Term Incentive Plan | Cash Awards
Common Stock Options Restricted Stock Warrants And Changes In Capitalization [Line Items]
Share-based compensation arrangement, conversion ratio to common stock	1

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