Business Combinations - Additional Information (Detail)					3 Months Ended		243 Months Ended
	Dec. 01, 2021 USD ($) Country BrokerageOperation Office	Nov. 09, 2021 USD ($)	May 20, 2021 USD ($)	May 17, 2021 USD ($)	Mar. 31, 2022 USD ($) Entity	Mar. 31, 2021 USD ($) Entity	Mar. 31, 2022 USD ($) Entity
Business Acquisition [Line Items]
Net cash raised through follow-on common stock offering				$ 1,437,900,000	$ 45,900,000	$ 37,200,000
Cash borrowed through debt issuance		$ 750,000,000
Accretion of the discount on acquisition					9,400,000	9,000,000.0
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations					$ (11,500,000)	$ (6,000,000.0)
Number of companies acquired | Entity					31	25	626
Aggregate amount of maximum earnout obligations related to acquisitions					$ 1,823,000,000.0
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet					962,300,000
Aggregate amount of earnout obligation expected settlement in cash or stock at option					646,500,000		$ 646,500,000
Aggregate amount of earnout obligation expected settlement in cash					315,800,000		315,800,000
Goodwill					68,900,000		68,900,000
Expiration lists					44,600,000		44,600,000
Non-compete agreements					4,100,000		4,100,000
Trade names					800,000		800,000
Total revenues					2,438,100,000	$ 2,171,000,000.0
Brokerage Management [Member]
Business Acquisition [Line Items]
Goodwill					68,900,000		68,900,000
Expiration lists					44,600,000		44,600,000
Non-compete agreements					4,100,000		4,100,000
Trade names					800,000		800,000
Brokerage and Risk Management [Member]
Business Acquisition [Line Items]
Write-off of amortizable intangible assets					200,000	$ 5,100,000
Brokerage [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					39,000,000.0		39,000,000.0
Brokerage [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					4,100,000		4,100,000
Brokerage [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					$ 800,000		$ 800,000
Minimum [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years					2 years
Minimum [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years					2 years
Minimum [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years					2 years
Maximum [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years					15 years
Maximum [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years					6 years
Maximum [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Estimated useful lives of intangibles assets, years					15 years
30-year Senior Note [Member]
Business Acquisition [Line Items]
Cash borrowed through debt issuance			$ 850,000,000
Term of notes			30 years
10-year Note [Member]
Business Acquisition [Line Items]
Cash borrowed through debt issuance		$ 400,000,000
Term of notes		10 years
30-year Note [Member]
Business Acquisition [Line Items]
Cash borrowed through debt issuance		$ 350,000,000
Term of notes		30 years
Willis Towers Watson PLC [Member]
Business Acquisition [Line Items]
Initial gross consideration	$ 3,250,000,000
Potential additional consideration subject to certain revenue targets	$ 750,000,000
Number of remaining international deferred closings | BrokerageOperation	1
Initial number of international deferred closings | BrokerageOperation	12
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					5.0		5.0
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					10.0		10.0
2022 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member]
Business Acquisition [Line Items]
Measurement input					9.0		9.0
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					4.7		4.7
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					5.2		5.2
2021 Acquisitions [Member] | Valuation, Income Approach | Minimum [Member]
Business Acquisition [Line Items]
Attrition rate					5.50%
2021 Acquisitions [Member] | Valuation, Income Approach | Maximum [Member]
Business Acquisition [Line Items]
Attrition rate					8.00%
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					11.0		11.0
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					16.5		16.5
Business Acquisition [Member]
Business Acquisition [Line Items]
Trade names					$ 800,000		$ 800,000
Annualized revenue of business acquisitions					32,200,000
Total revenues					5,100,000
Net earnings					600,000
Business Acquisition [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists					44,600,000		44,600,000
Non-compete agreements					4,100,000		4,100,000
Deferred tax liability					$ 10,200,000		$ 10,200,000
Gallagher Re [Member]
Business Acquisition [Line Items]
Number of operating offices | Office	70
Number of operating countries | Country	31
Number of employees | Country	2,200