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Employee Benefits (Tables) | 9 Months Ended
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Sep. 30, 2025
Retirement Benefits [Abstract] |
Schedule of Components of Net Periodic Benefit Cost for the Pension Plans | The following table provides the components of the net periodic (benefit) cost recognized in the Condensed Consolidated Statements of Income for Aon’s significant U.K., U.S., and other major pension plans, which are located in the Netherlands and Canada. Service cost is reported in Compensation and benefits and all other components are reported in Other income (expense) as follows (in millions):
| Three Months Ended September 30,
| U.K. |  | U.S. |  | Other
| 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Interest cost | 38 |  |  | 37 |  |  | 26 |  |  | 23 |  |  | 10 |  |  | 10 |
Expected return on plan assets, net of administration expenses | (46) |  |  | (48) |  |  | (30) |  |  | (33) |  |  | (13) |  |  | (14) |
Amortization of prior-service cost | 1 |  |  | 1 |  |  | — |  |  | — |  |  | — |  |  | — |
Amortization of net actuarial loss | 23 |  |  | 21 |  |  | 8 |  |  | 7 |  |  | 4 |  |  | 4 |
Net periodic cost (benefit) | $ | 16 |  |  | $ | 11 |  |  | $ | 4 |  |  | $ | (3) |  |  | $ | 1 |  |  | $ | — |
| Nine Months Ended September 30,
| U.K. |  | U.S. |  | Other
| 2025 |  | 2024 |  | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Interest cost | 114 |  |  | 108 |  |  | 77 |  |  | 69 |  |  | 29 |  |  | 29 |
Expected return on plan assets, net of administration expenses | (134) |  |  | (142) |  |  | (90) |  |  | (101) |  |  | (39) |  |  | (41) |
Amortization of prior-service cost | 2 |  |  | 2 |  |  | — |  |  | — |  |  | — |  |  | — |
Amortization of net actuarial loss | 67 |  |  | 62 |  |  | 26 |  |  | 22 |  |  | 11 |  |  | 10 |
Net periodic (benefit) cost | $ | 49 |  |  | $ | 30 |  |  | $ | 13 |  |  | $ | (10) |  |  | $ | 1 |  |  | $ | (2) |
Schedule of Employer's Contributions | The following table summarizes contributions made to the Company’s significant pension plans (in millions):
| Three Months Ended September 30, |  | Nine Months Ended September 30,
| 2025 |  | 2024 |  | 2025 |  | 2024
Contributions to U.K. pension plans | $ | 1 |  |  | $ | 1 |  |  | $ | 3 |  |  | $ | 3 |
Contributions to U.S. pension plans | 23 |  |  | 13 |  |  | 67 |  |  | 31 |
Contributions to other major pension plans | 2 |  |  | 2 |  |  | 10 |  |  | 9 |
Total contributions | $ | 26 |  |  | $ | 16 |  |  | $ | 80 |  |  | $ | 43 |

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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (q) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (h) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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