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Basis of Presentation, Significant Accounting Policies and Recent Accounting Pronouncements - Narrative (Details) $ in Millions		12 Months Ended
	Jul. 26, 2021 USD ($)	Dec. 31, 2023 USD ($) Unit	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Unencumbered and available funds required		$ 1,424	$ 1,262	$ 4,486
Percentage of lease denominated in U.S. dollars, Pounds Sterling or Euros		90.00%
Number of reporting units \| Unit		7
Termination Of Proposed Combination With A O N Plc [Member] \| Contract Termination [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Termination payment	$ 1,000			$ 1,000
Other [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		6.00%	6.00%	6.00%
Reimbursed Expenses [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		1.00%	1.00%	1.00%
Buildings [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Useful life		50 years
Minimum [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Percentage of entity voting rights		50.00%
Real estate lease term		3 years
Minimum [Member] \| Broking [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		47.00%	47.00%	47.00%
Minimum [Member] \| Consulting [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		32.00%	32.00%	32.00%
Minimum [Member] \| Outsourced administration [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		12.00%	12.00%	12.00%
Minimum [Member] \| Internally developed software [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Useful life		3 years
Minimum [Member] \| Furniture, equipment and software [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Useful life		3 years
Minimum [Member] \| Willis Limited [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Unencumbered and available funds required		$ 105
Unencumbered and available cash balance required		$ 66
Maximum [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Real estate lease term		11 years
Term for renewal commissions expected		25 years
Maximum [Member] \| Broking [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		48.00%	48.00%	48.00%
Maximum [Member] \| Consulting [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		33.00%	33.00%	33.00%
Maximum [Member] \| Outsourced administration [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Portion of Revenue		13.00%	13.00%	13.00%
Maximum [Member] \| Internally developed software [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Useful life		10 years
Maximum [Member] \| Furniture, equipment and software [Member]
Significant Accounting Policies And Recent Accounting Pronouncements [Line Items]
Useful life		10 years