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Employee Benefits (Tables)
 12 Months Ended
Dec. 31, 2024
Retirement Benefits [Abstract]  
Schedule of Expense Recognized for Defined Contribution Savings Plans, Included in Compensation and Benefits and Discontinued Operations in the Consolidated Statements of Income The expense for the significant plans in the U.S., U.K., Netherlands, and Canada is as follows (in millions):
Years Ended December 31
202420232022
U.S.$124 $114 $108 
U.K.55 52 47 
Netherlands and Canada33 33 33 
Total$212 $199 $188 
Schedule of Changes in Projected Benefit Obligations Fair Value of Plan Assets, Funded Status and Amount Recognized
The following tables provide a reconciliation of the changes in the projected benefit obligations and fair value of assets for the years ended December 31, 2024 and 2023, and a statement of the funded status as of December 31, 2024 and 2023, for Aon’s significant U.K., U.S., and other major pension plans, which are located in the Netherlands and Canada. These plans represent approximately 87% of the Company’s projected benefit obligations.
 U.K.U.S.Other
(millions)202420232024202320242023
Change in projected benefit obligation    
At January 1$3,233 $2,946 $2,199 $2,180 $1,129 $1,094 
Service cost— — — — — — 
Interest cost145 147 100 103 39 41 
Plan amendment7 5 — — — — 
Settlements— — — 3 — (63)
Actuarial (gain) loss(353)143 (116)51 36 63 
Benefit payments(176)(173)(143)(138)(44)(44)
Foreign currency impact(43)165 — — (73)38 
As of December 31$2,813 $3,233 $2,040 $2,199 $1,087 $1,129 
Accumulated benefit obligation at end of year$2,813 $3,233 $2,040 $2,199 $1,077 $1,117 
Change in fair value of plan assets   
At January 1$3,775 $3,537 $1,499 $1,481 $1,069 $1,032 
Actual return on plan assets(261)211 26 121 65 93 
Employer contributions4 4 43 32 11 14 
Settlements— — — 3 — (63)
Benefit payments(176)(173)(143)(138)(44)(44)
Foreign currency impact(51)196 — — (69)37 
As of December 31$3,291 $3,775 $1,425 $1,499 $1,032 $1,069 
Market related value at end of year$3,291 $3,775 $1,731 $1,788 $1,032 $1,069 
Amount recognized in Statement of Financial Position as of December 31   
Funded status$478 $542 $(615)$(700)$(55)$(60)
Unrecognized prior-service cost43 39 — — (5)(5)
Unrecognized loss1,854 1,870 1,276 1,319 419 436 
Net amount recognized$2,375 $2,451 $661 $619 $359 $371 
The following table provides an overview of the accumulated PBO, fair value of plan assets, funded status and net amount recognized as of December 31, 2024 and 2023 for the Company’s other significant postretirement benefit plans located in the U.S. and Canada (in millions):
20242023
Accumulated projected benefit obligation$79 $91 
Fair value of plan assets16 15 
Funded status(63)(76)
Unrecognized (gain) loss(26)(16)
Net amount recognized$(89)$(92)
Schedule of Amounts Recognized in the Consolidated Statements of Financial Position
Amounts recognized in the Consolidated Statements of Financial Position consist of (in millions):
 U.K.U.S.Other
 202420232024202320242023
Prepaid benefit cost (1)
$505 $570 $— $— $— $1 
Accrued benefit liability - current (2)
(1)(1)(42)(42)(4)(4)
Accrued benefit liability - non-current (3)
(26)(27)(573)(658)(51)(57)
Accumulated other comprehensive loss 1,897 1,909 1,276 1,319 414 431 
Net amount recognized$2,375 $2,451 $661 $619 $359 $371 
(1)Included in Prepaid pension.
(2)Included in Other current liabilities.
(3)Included in Pension, other postretirement, and postemployment liabilities.
Schedule of Amounts Recognized in Accumulated Other Comprehensive Loss that have not yet been Recognized as Components of net Periodic Benefit Cost
Amounts recognized in Accumulated other comprehensive loss (income) that have not yet been recognized as components of net periodic benefit cost at December 31, 2024 and 2023 consist of (in millions):
 U.K.U.S.Other
 202420232024202320242023
Net loss$1,854 $1,870 $1,276 $1,319 $419 $436 
Prior service cost (income)43 39 — — (5)(5)
Total$1,897 $1,909 $1,276 $1,319 $414 $431 
Schedule of Components of Net Periodic Benefit Cost for the Pension Plans
Service cost is reported in Compensation and benefits and all other components are reported in Other income (expense) as follows (in millions):
 U.K.U.S.Other
 202420232022202420232022202420232022
Service cost$— $— $1 $— $— $— $— $— $— 
Interest cost145 147 83 100 103 73 39 41 19 
Expected return on plan assets, net of administration expenses(188)(190)(134)(135)(119)(108)(54)(48)(33)
Amortization of prior-service cost2 2 2 — — — — — — 
Amortization of net actuarial loss82 75 29 36 34 61 13 13 13 
Net periodic benefit (income) cost41 34 (19)1 18 26 (2)6 (1)
Settlement expense— — — — — 170 — 27 — 
Total net periodic benefit cost (income)$41 $34 $(19)$1 $18 $196 $(2)$33 $(1)
Schedule of Weighted-Average Assumptions Used to Determine Future Benefit Obligations and Net Periodic Benefit Cost
The weighted-average assumptions used to determine benefit obligations are as follows:
 U.K.
U.S. (1)
Other
 202420232024202320242023
Discount rate
5.55%
4.58%
5.18 - 5.52%
4.60 - 4.84%
3.26 - 4.63%
2.95 - 4.65%
Rate of compensation increase
3.44 - 3.94%
3.38 - 3.88%
N/A
N/A
1.00 - 3.00%
1.00 - 3.00%
Underlying price inflation2.44%2.33%N/AN/A2.00%2.00%
(1)U.S. pension plans are frozen and therefore not impacted by compensation increases or price inflation.
The weighted-average assumptions used to determine the net periodic benefit cost are as follows:
 U.K.U.S.Other
 202420232022202420232022202420232022
Discount rate
4.55%
4.95%
1.85%
4.65 - 4.72%
4.80 - 4.91%
1.67 - 2.25%
3.04 - 4.63%
3.35 - 5.15%
0.84 - 2.58%
Expected return on plan assets, net of administration expenses
5.14%
5.34%
2.34%
7.79%
6.82%
2.03 - 5.28%
4.40 - 5.50%
4.20 - 4.85%
1.80 - 3.15%
Rate of compensation increase
3.38 - 3.88%
3.59 - 4.09%
3.62 - 4.12%
N/A
N/A
N/A
1.00 - 3.00%
1.00 - 3.00%
1.00 - 3.00%
Schedule of Fair Values of Pension Plan Assets
The fair values of the Company’s U.S. pension plan assets at December 31, 2024 and December 31, 2023, by asset category, are as follows (in millions):
  Fair Value Measurements Using
Asset CategoryBalance at December 31, 2024Quoted Prices in Active Markets for Identical Assets
(Level 1)		Significant Other Observable Inputs
(Level 2)		Significant Unobservable Inputs
(Level 3)
Cash and cash equivalents (1)
$14 $14 $— $— 
Equity investments:  
Equity securities— — — — 
Equity derivatives— — — — 
Pooled funds (2)
427 — — — 
Fixed income investments: 
Corporate bonds92 — 92 — 
Government and agency bonds252 218 34 — 
Fixed Income Derivatives3 — 3 — 
Pooled funds (2)
339 — — — 
Other investments: 
Real estate (2) (3)
127 — — — 
Alternative investments (2) (4)
171 — — — 
Total$1,425 $232 $129 $— 
  Fair Value Measurements Using
Asset CategoryBalance at December 31, 2023Quoted Prices in Active Markets for Identical Assets
(Level 1)		Significant Other Observable Inputs
(Level 2)		Significant Unobservable Inputs
(Level 3)
Cash and cash equivalents (1)
$35 $35 $— $— 
Equity investments:
Equity securities74 74 — — 
Equity derivatives— — — — 
Pooled funds (2)
341 — — — 
Fixed income investments:
Corporate bonds124 — 124 — 
Government and agency bonds272 232 40 — 
Fixed Income Derivatives2 — 2 — 
Pooled funds (2)
373 — — — 
Other investments:
Real estate (2)(3)
93 — — — 
Alternative investments (2) (4)
185 — — — 
Total$1,499 $341 $166 $— 
(1)Consists of cash and institutional short-term investment funds.
(2)Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note.
(3)Consists of property funds and trusts holding direct real estate investments.
(4)Consists of limited partnerships, private equity, and hedge funds.
The fair values of the Company’s major U.K. pension plan assets at December 31, 2024 and December 31, 2023, by asset category, are as follows (in millions):
  Fair Value Measurements Using
 Balance at December 31, 2024Quoted Prices in Active Markets for Identical Assets
(Level 1)		Significant Other Observable Inputs
(Level 2)		Significant Unobservable Inputs
(Level 3)
Cash and cash equivalents (1)
$98 $98 $— $— 
Equity investments: 
Pooled funds (2)
— — — — 
Fixed income investments: 
Derivatives (3)
(129)— (129)— 
Government and agency bonds1,397 1,397 — — 
Annuities1,329 — — 1,329 
Pooled funds (2)
168 — — — 
Other investments:
Real estate (2) (4)
58 — — — 
Pooled funds (2) (5)
370 — — — 
Total$3,291 $1,495 $(129)$1,329 
  Fair Value Measurements Using
 Balance at December 31, 2023Quoted Prices in Active Markets for Identical Assets
(Level 1)		Significant Other Observable Inputs
(Level 2)		Significant Unobservable Inputs
(Level 3)
Cash and cash equivalents (1)
$273 $273 $— $— 
Equity investments:
Pooled funds (2)
— — — — 
Fixed income investments:
Derivatives (3)
(255)— (255)— 
Government and agency bonds1,570 1,570 — — 
Annuities1,510 — — 1,510 
Pooled funds (2)
160 — — — 
Other investments:
Real estate (2) (4)
92— — — 
Pooled funds (2) (5)
425 — — — 
Total$3,775 $1,843 $(255)$1,510 
(1)Consists of cash and institutional short-term investment funds.
(2)Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note.
(3)Consists of equity securities and equity derivatives, including repurchase agreements.
(4)Consists of property funds and trusts holding direct real estate investments.
(5)Consists of multi-strategy limited partnerships, private equity, hedge funds, and collective investment schemes with a diversified portfolio of cash, equities, equity related securities, derivatives, and/or fixed income securities.
The fair values of the Company’s other major pension plan assets at December 31, 2024 and December 31, 2023, by asset category, are as follows (in millions):
  Fair Value Measurements Using
 Balance at December 31, 2024Quoted Prices in Active Markets for Identical Assets
(Level 1)		Significant Other Observable Inputs
(Level 2)		Significant Unobservable Inputs
(Level 3)
Cash and cash equivalents (1)
$18 $18 $— $— 
Equity investments: 
Equity securities55 55 — — 
Pooled funds (2)
208 — — — 
Fixed income investments: 
Government and agency bonds237 237 — — 
Derivatives(12)— (12)— 
Pooled funds (2)
473 — — — 
Other investments: 
Alternative investments (2) (3)
47 — — — 
Real estate (2) (4)
6 — — — 
Total$1,032 $310 $(12)$— 
  Fair Value Measurements Using
 Balance at December 31, 2023Quoted Prices in Active Markets for Identical Assets
(Level 1)		Significant Other Observable Inputs
(Level 2)		Significant Unobservable Inputs
(Level 3)
Cash and cash equivalents (1)
$26 $26 $— $— 
Equity investments:
Equity securities53 53 — — 
Pooled funds (2)
204 — — — 
Fixed income investments:
Government and agency bonds243 243 — — 
Derivatives(3)— (3)
Pooled funds (2)
490 — — — 
Other investments:
Alternative investments (2) (3)
47 — — — 
Real estate (2) (4)
9 — — — 
Total$1,069 $322 $(3)$— 
(1)Consists of cash and institutional short-term investment funds.
(2)Certain investments measured at fair value using the net asset value per share practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in the above table are intended to permit reconciliation of the fair values to the amounts presented in the plan assets contained in this Note.
(3)Consists of limited partnerships, private equity, and hedge funds.
(4)Consists of property funds and trusts holding direct real estate investments.
Schedule of Changes in the Level 3 Fair-Value Category
The following table presents the changes in the Level 3 fair-value category in the Company’s U.K. pension plans for the years ended December 31, 2024 and December 31, 2023 (in millions):
Fair Value Measurements Using Level 3 InputsAnnuities
Balance at January 1, 2023$1,403 
Actual return on plan assets:
Relating to assets still held at December 31, 202330 
Foreign exchange77 
Balance at December 31, 20231,510 
Actual return on plan assets:
Relating to assets still held at December 31, 2024(157)
Foreign exchange(24)
Balance at December 31, 2024$1,329 
Schedule of Estimated Future Benefit Payments
Estimated future benefit payments for plans, not including voluntary one-time lump sum payments, are as follows at December 31, 2024 (in millions):
U.K.U.S.Other
2025$167 $163 $45 
2026$172 $166 $46 
2027$177 $167 $48 
2028$182 $168 $49 
2029$186 $158 $50 
2030 - 2034$974 $747 $267 
Schedule of Other Information Related to the Company's Other Post-Retirement Benefit Plans
Other information related to the Company’s other postretirement benefit plans are as follows:
202420232022
Net periodic benefit cost recognized (millions)$4$4$3
Weighted-average discount rate used to determine future benefit obligations
4.68 - 5.59%
4.65 -4.87%
4.94 -5.19%
Weighted-average discount rate used to determine net periodic benefit costs
4.64 - 4.76%
4.92 -5.17%
1.97 - 2.69%