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Business Combinations - Additional Information (Detail) |  |  |  |  | 12 Months Ended
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Dec. 01, 2021 USD ($) Country BrokerageOperation Office Employee | Nov. 09, 2021 USD ($) | May 20, 2021 USD ($) | May 17, 2021 USD ($) | Dec. 31, 2021 USD ($) Entity | Dec. 31, 2020 USD ($) Entity | Dec. 31, 2019 USD ($) Entity
Business Acquisition [Line Items] |  |  |  |  |  |  |
Net cash raised through follow-on public offering common stock |  |  |  | $ 1,437,900,000 | $ 1,546,700,000 | $ 111,900,000 | $ 101,200,000
Cash borrowed through debt issuance |  | $ 750,000,000 |  |  |  |  |
Accretion of the discount on acquisition |  |  |  |  | 35,700,000 | 32,500,000 | 27,000,000.0
Income (expense) related to net adjustments to estimated fair value of liability for earnout obligations |  |  |  |  | $ (83,900,000) | $ 65,400,000 | $ 11,700,000
Number of companies acquired | Entity |  |  |  |  | 99 | 135 | 116
Aggregate amount of maximum earnout obligations related to acquisitions |  |  |  |  | $ 1,873,900,000 | $ 1,128,100,000 |
Aggregate amount of maximum earnout obligations related to acquisitions, recorded in consolidated balance sheet |  |  |  |  | 988,500,000 | 592,200,000 |
Aggregate amount of earnout obligation expected settlement in cash |  |  |  |  | 318,200,000 | 98,500,000 |
Aggregate amount of earnout obligation expected settlement in cash or stock at option |  |  |  |  | 670,300,000 | 493,700,000 |
Goodwill |  |  |  |  | 2,561,500,000 |  |
Expiration lists |  |  |  |  | 2,027,800,000 |  |
Non-compete agreements |  |  |  |  | 7,300,000 |  |
Trade names |  |  |  |  | 5,200,000 |  |
Write-off of amortizable intangible assets |  |  |  |  | 17,600,000 | 51,700,000 | $ 100,000
Total revenues |  |  |  |  | 9,065,500,000 | 7,979,400,000 |
Brokerage Management [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Goodwill |  |  |  |  | 2,561,500,000 |  |
Expiration lists |  |  |  |  | 2,027,800,000 |  |
Non-compete agreements |  |  |  |  | 7,300,000 |  |
Trade names |  |  |  |  | 5,200,000 |  |
Brokerage and Risk Management [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Write-off of amortizable intangible assets |  |  |  |  | 17,600,000 | $ 51,700,000 | $ 100,000
Brokerage [Member] | Expiration Lists [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 393,000,000.0 |  |
Brokerage [Member] | Non-Compete Agreements [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | 3,900,000 |  |
Brokerage [Member] | Trade Names [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Business acquisition not deductible for income tax purposes |  |  |  |  | $ 3,800,000 |  |
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 |  |  |  |  |
Gallagher Re [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of operating offices | Office | 70 |  |  |  |  |  |
Number of operating countries | Country | 31 |  |  |  |  |  |
Number of employees | Employee | 2,200 |  |  |  |  |  |
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 | 12 |  |  |  |  |  |
Initial purchase consideration subject to local regulatory approval | $ 180,000,000 |  |  |  |  |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Attrition rate |  |  |  |  | 4.30% |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Attrition rate |  |  |  |  | 16.10% |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 2.5 |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 15.0 |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 3.0 |  |
2021 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 4.8 |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 7.0 |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 10.5 |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 9.0 |  |
2021 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 17.0 |  |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Attrition rate |  |  |  |  | 4.30% |  |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Attrition rate |  |  |  |  | 16.10% |  |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 3.0 |  |
2020 Acquisitions [Member] | Valuation, Market Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 4.8 |  |
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Minimum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 9.0 |  |
2020 Acquisitions [Member] | Valuation, Income Approach [Member] | Measurement Input, Long-term Revenue Growth Rate [Member] | Maximum [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Measurement input |  |  |  |  | 17.0 |  |
Business Acquisition [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Trade names |  |  |  |  | $ 5,200,000 |  |
Annualized revenue of business acquisitions |  |  |  |  | 1,002,000,000.0 |  |
Total revenues |  |  |  |  | 188,200,000 |  |
Net earnings |  |  |  |  | (13,000,000.0) |  |
Business Acquisition [Member] | Brokerage [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Expiration lists |  |  |  |  | 2,027,800,000 |  |
Non-compete agreements |  |  |  |  | 7,300,000 |  |
Deferred tax liability |  |  |  |  | $ 89,800,000 |  |

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