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Employee Benefits (Tables)
 12 Months Ended
Dec. 31, 2023
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
202320222021
U.S.$114 $108 $103 
U.K.52 47 46 
Netherlands and Canada33 33 35 
Total$199 $188 $184 
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, 2023 and 2022, and a statement of the funded status as of December 31, 2023 and 2022, 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 88% of the Company’s projected benefit obligations.
 U.K.U.S.Other
(millions)202320222023202220232022
Change in projected benefit obligation    
At January 1$2,946 $4,919 $2,180 $3,164 $1,094 $1,531 
Service cost— 1 — — — — 
Interest cost147 83 103 73 41 19 
Plan amendment5 — — — — — 
Settlements— — 3 (283)(63)— 
Actuarial (gain) loss143 (1,418)51 (607)63 (322)
Benefit payments(173)(195)(138)(167)(44)(45)
Foreign currency impact165 (444)— — 38 (89)
As of December 31$3,233 $2,946 $2,199 $2,180 $1,129 $1,094 
Accumulated benefit obligation at end of year$3,233 $2,946 $2,199 $2,180 $1,117 $1,079 
Change in fair value of plan assets   
At January 1$3,537 $6,246 $1,481 $2,378 $1,032 $1,430 
Actual return on plan assets211 (1,961)121 (484)93 (284)
Employer contributions4 7 32 37 14 15 
Settlements— — 3 (283)(63)— 
Benefit payments(173)(195)(138)(167)(44)(45)
Foreign currency impact196 (559)— — 37 (83)
As of December 31$3,775 $3,537 $1,499 $1,481 $1,069 $1,032 
Market related value at end of year$3,775 $3,537 $1,788 $1,794 $1,069 $1,032 
Amount recognized in Statement of Financial Position as of December 31   
Funded status$542 $591 $(700)$(699)$(60)$(62)
Unrecognized prior-service cost39 35 — — (5)(6)
Unrecognized loss1,870 1,726 1,319 1,305 436 443 
Net amount recognized$2,451 $2,352 $619 $606 $371 $375 
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, 2023 and 2022 for the Company’s other significant postretirement benefit plans located in the U.S. and Canada (in millions):
20232022
Accumulated projected benefit obligation$91 $83 
Fair value of plan assets15 14 
Funded status(76)(69)
Unrecognized prior-service credit— (1)
Unrecognized (gain) loss(16)(19)
Net amount recognized$(92)$(89)
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
 202320222023202220232022
Prepaid benefit cost (1)
$570 $612 $— $— $1 $— 
Accrued benefit liability - current (2)
(1)(1)(42)(42)(4)(5)
Accrued benefit liability - non-current (3)
(27)(20)(658)(657)(57)(57)
Accumulated other comprehensive loss 1,909 1,761 1,319 1,305 431 437 
Net amount recognized$2,451 $2,352 $619 $606 $371 $375 
(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, 2023 and 2022 consist of (in millions):
 U.K.U.S.Other
 202320222023202220232022
Net loss$1,870 $1,726 $1,319 $1,305 $436 $443 
Prior service cost (income)39 35 — — (5)(6)
Total$1,909 $1,761 $1,319 $1,305 $431 $437 
Schedule of Components of Net Periodic Benefit Cost for the Pension Plans
Service cost is reported in Compensation and benefits and all other components are reported in Other income (expense) as follows (in millions):
 U.K.U.S.Other
 202320222021202320222021202320222021
Service cost$— $1 $1 $— $— $— $— $— $— 
Interest cost147 83 65 103 73 57 41 19 12 
Expected return on plan assets, net of administration expenses(190)(134)(137)(119)(108)(130)(48)(33)(32)
Amortization of prior-service cost2 2 2 — — — — — — 
Amortization of net actuarial loss75 29 32 34 61 78 13 13 15 
Net periodic benefit (income) cost34 (19)(37)18 26 5 6 (1)(5)
Settlement expense— — 5 — 170 — 27 — — 
Total net periodic benefit cost (income)$34 $(19)$(32)$18 $196 $5 $33 $(1)$(5)
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
 202320222023202220232022
Discount rate
4.58%
4.89%
4.60 - 4.84%
4.82 - 5.03%
2.95 - 4.65%
3.23 - 5.19%
Rate of compensation increase
3.38 - 3.88%
3.59 - 4.09%
N/A
N/A
1.00 - 3.00%
1.00 - 3.00%
Underlying price inflation2.33%2.44%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
 202320222021202320222021202320222021
Discount rate
4.95%
1.85%
1.20%
4.80 - 4.91%
1.67 - 2.25%
1.12 - 1.79%
3.35 - 5.15%
0.84 - 2.58%
0.28 - 2.00%
Expected return on plan assets, net of administration expenses
5.34%
2.34%
2.04%
6.82%
2.03 - 5.28%
2.65 - 6.56%
4.20 - 4.85%
1.80 - 3.15%
1.70 - 2.65%
Rate of compensation increase
3.59 - 4.09%
3.62 - 4.12%
3.22 - 3.72%
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, 2023 and December 31, 2022, by asset category, are as follows (in millions):
  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 $— 
  Fair Value Measurements Using
Asset CategoryBalance at December 31, 2022Quoted 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)
$42 $42 $— $— 
Equity investments:
Equity securities64 64 — — 
Equity derivatives(6)— (6)— 
Pooled funds (2)
293 — — — 
Fixed income investments:
Corporate bonds192 — 192 — 
Government and agency bonds149 124 25 — 
Pooled funds (2)
507 — — — 
Other investments:
Real estate (2)(3)
113 — — — 
Alternative investments (2) (4)
127 — — — 
Total$1,481 $230 $211 $— 
(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, 2023 and December 31, 2022, by asset category, are as follows (in millions):
  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 
  Fair Value Measurements Using
 Balance at December 31, 2022Quoted 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)
$336 $336 $— $— 
Equity investments:
Pooled funds (2)
— — — — 
Fixed income investments:
Derivatives (3)
(548)— (548)— 
Government and agency bonds1,593 1,593 — — 
Annuities1,403 — — 1,403 
Pooled funds (2)
139 — — — 
Other investments:
Real estate (2) (4)
97— — — 
Pooled funds (2) (5)
517 — — — 
Total$3,537 $1,929 $(548)$1,403 
(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, 2023 and December 31, 2022, by asset category, are as follows (in millions):
  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 $(3)$— 
  Fair Value Measurements Using
 Balance at December 31, 2022Quoted 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)
$30 $30 $— $— 
Equity investments:
Equity securities53 53 — — 
Pooled funds (2)
211 — — — 
Fixed income investments:
Government and agency bonds225 225 — — 
Derivatives(3)— (3)
Pooled funds (2)
462 — — — 
Other investments:
Alternative investments (2) (3)
42 — — — 
Real estate (2) (4)
12 — — — 
Total$1,032 $308 $(3)$— 
(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, 2023 and December 31, 2022 (in millions):
Fair Value Measurements Using Level 3 InputsAnnuities
Balance at January 1, 2022$2,305 
Actual return on plan assets:
Relating to assets still held at December 31, 2022(674)
Purchase, sales and settlements-net— 
Foreign exchange(228)
Balance at December 31, 20221,403 
Actual return on plan assets:
Relating to assets still held at December 31, 202330 
Purchases, sales and settlements-net— 
Foreign exchange77 
Balance at December 31, 2023$1,510 
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, 2023 (in millions):
U.K.U.S.Other
2024$165 $160 $47 
2025$169 $159 $48 
2026$174 $164 $50 
2027$179 $166 $51 
2028$184 $159 $52 
2029 - 2033$966 $756 $275 
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:
202320222021
Net periodic benefit cost recognized (millions)$4$3$5
Weighted-average discount rate used to determine future benefit obligations
4.65 - 4.87%
4.94 -5.19%
2.52 -3.06%
Weighted-average discount rate used to determine net periodic benefit costs
4.92 - 5.17%
1.97 -2.69%
1.45 - 2.68%