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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 |  |  |  |  |  |

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