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Business Combinations - Additional Information (Detail) |  |  |  |  | 3 Months Ended | 6 Months Ended | 246 Months Ended |
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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 |  |  |  |  |  |  |  |  |

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