Business Combinations - Additional Information (Detail)					3 Months Ended		6 Months Ended		246 Months Ended
	Dec. 01, 2021 USD ($) Office Entity Employee BrokerageOperation	Nov. 09, 2021 USD ($)	May 20, 2021 USD ($)	May 17, 2021 USD ($)	Jun. 30, 2022 USD ($) Entity	Jun. 30, 2021 USD ($) Entity	Jun. 30, 2022 USD ($) Entity	Jun. 30, 2021 USD ($) Entity	Jun. 30, 2022 USD ($) Entity	Dec. 31, 2021 USD ($)
Business Acquisition [Line Items]
Net cash raised through follow-on common stock offering				$ 1,437,900,000			$ 69,100,000	$ 1,502,000,000.0
Cash borrowed through debt issuance		$ 750,000,000
Accretion of the discount on acquisition					$ 14,000,000.0	$ 9,200,000	23,400,000	18,200,000
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations					$ 47,800,000	$ 4,600,000	$ 36,300,000	$ (10,600,000)
Number of companies acquired | Entity					30	26	50	48	635
Aggregate amount of maximum earnout obligations related to acquisitions							$ 1,753,200,000
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet							848,700,000
Aggregate amount of earnout obligation expected settlement in cash or stock at option					$ 588,300,000		588,300,000		$ 588,300,000
Aggregate amount of earnout obligation expected settlement in cash					260,400,000		260,400,000		260,400,000
Goodwill					213,600,000		213,600,000		213,600,000
Expiration lists					169,800,000		169,800,000		169,800,000
Goodwill					9,036,400,000		9,036,400,000		9,036,400,000	$ 8,666,200,000
Non-compete agreements					10,800,000		10,800,000		10,800,000
Trade names					1,700,000		1,700,000		1,700,000
Total revenues					2,048,500,000	$ 1,953,800,000	4,501,800,000	$ 4,137,200,000
Brokerage Management [Member]
Business Acquisition [Line Items]
Goodwill					213,600,000		213,600,000		213,600,000
Expiration lists					169,800,000		169,800,000		169,800,000
Non-compete agreements					10,800,000		10,800,000		10,800,000
Trade names					1,700,000		1,700,000		1,700,000
Brokerage and Risk Management [Member]
Business Acquisition [Line Items]
Write-off of amortizable intangible assets							400,000	$ 13,100,000
Brokerage [Member]
Business Acquisition [Line Items]
Goodwill					8,911,800,000		8,911,800,000		8,911,800,000	$ 8,544,600,000
Brokerage [Member] | Expiration Lists [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					101,000,000.0		101,000,000.0		101,000,000.0
Brokerage [Member] | Non-Compete Agreements [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					10,200,000		10,200,000		10,200,000
Brokerage [Member] | Trade Names [Member]
Business Acquisition [Line Items]
Business acquisition not deductible for income tax purposes					1,700,000		$ 1,700,000		1,700,000
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
Revision of Prior Period, Adjustment [Member]
Business Acquisition [Line Items]
Expiration lists					(608,000,000.0)		$ (608,000,000.0)		(608,000,000.0)
Increase in acquired software					59,100,000		59,100,000		59,100,000
Fair value of earnout liability					(1,600,000)		(1,600,000)		(1,600,000)
Working capital adjustment					121,400,000		121,400,000		121,400,000
Goodwill					$ 425,900,000		$ 425,900,000		$ 425,900,000
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					3.0		3.0		3.0
2022 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]
Business Acquisition [Line Items]
Measurement input					14.0		14.0		14.0
2022 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Discount Rate [Member]
Business Acquisition [Line Items]
Measurement input					9.0		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					3.2		3.2		3.2
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		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							9.50%
2021 Acquisitions [Member] | Valuation, Income Approach | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member]
Business Acquisition [Line Items]
Measurement input					10.0		10.0		10.0
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		16.5
Business Acquisition [Member]
Business Acquisition [Line Items]
Annualized revenue of business acquisitions							$ 84,800,000
Total revenues							18,700,000
Net earnings							(1,700,000)
Business Acquisition [Member] | Brokerage [Member]
Business Acquisition [Line Items]
Expiration lists					$ 169,800,000		169,800,000		$ 169,800,000
Non-compete agreements					10,800,000		10,800,000		10,800,000
Deferred tax liability					$ 22,500,000		$ 22,500,000		$ 22,500,000
Gallagher Re [Member]
Business Acquisition [Line Items]
Number of operating offices | Office	70
Number of operating countries | Entity	31
Number of employees | Employee	2,200