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Employee Benefits
 12 Months Ended
Dec. 31, 2023
Retirement Benefits [Abstract]  
Employee Benefits Employee Benefits
Defined Contribution Savings Plans
Aon maintains defined contribution savings plans for the benefit of its employees. The expense recognized for these plans is included in Compensation and benefits 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 
Pension and Other Postretirement Benefits
The Company sponsors defined benefit pension and postretirement health and welfare plans that provide retirement, medical, and life insurance benefits. The postretirement health care plans are contributory, with retiree contributions adjusted annually, and the life insurance and pension plans are generally noncontributory. The significant U.S., U.K., Netherlands, and Canada pension plans are closed to new entrants.
Pension Plans
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 
In 2023, the net actuarial losses increased the benefit obligation primarily due to the decrease in discount rates. During 2022, the benefit obligation decreased primarily as a result of increases in the discount rates. In 2022, the value of plan assets decreased as a result of negative asset returns, predominately in the U.K, and overall weaker exchange rates versus the U.S. Dollar compared to prior year. The U.K. pension assets are concentrated in fixed income and annuities which were disproportionately negatively impacted by the decreasing interest rate environment. The U.K. hedging strategy continues to perform as expected and the related pension plans remain over-funded.

In May 2023, to further its pension de-risking strategy, the Company settled certain pension obligations in the Netherlands through the purchase of annuities, where certain pension assets were liquidated to purchase the annuities. The transaction settled $63 million of benefit obligations using $63 million of assets.

In November 2022, to further its pension de-risking strategy the Company purchased an annuity for portions of its U.S. pension plans that will settle certain obligations. The transaction settled $280 million of benefit obligations using $280 million of assets.
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.
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 
In 2023, U.S. plans with a PBO and an ABO in excess of the fair value of plan assets had a PBO of $2.2 billion, an ABO of $2.2 billion, and plan assets with a fair value of $1.5 billion. U.K. plans with a PBO and an ABO in excess of the fair value of plan assets had a PBO of $124 million, an ABO of $124 million and, plan assets with a fair value of $96 million. Other plans with a PBO in excess of the fair value of plan assets had a PBO of $1.0 billion and plan assets with a fair value of $0.9 billion, and other plans with an ABO in excess of the fair value of plan assets had an ABO of $233 million and plan assets with a fair value of $178 million.
In 2022, U.S. plans with a PBO and an ABO in excess of the fair value of plan assets had a PBO of $2.2 billion, an ABO of $2.2 billion, and plan assets with a fair value of $1.5 billion. U.K. plans with a PBO and an ABO in excess of the fair value of plan assets had a PBO of $114 million, an ABO of $114 million and, plan assets with a fair value of $93 million. Other plans with a PBO in excess of the fair value of plan assets had a PBO of $1.1 billion and plan assets with a fair value of $1.0 billion, and other plans with an ABO in excess of the fair value of plan assets had an ABO of $220 million and plan assets with a fair value of $116 million.
Service cost is reported in Compensation and benefits and all other components are reported in Other income (expense) as follows (in millions):
 U.K.U.S.Other
 202320222021202320222021202320222021
Service cost$— $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)
The Company uses a full-yield curve approach in the estimation of the service and interest cost components of net periodic pension and postretirement benefit cost for its major pension and other postretirement benefit plans. This estimation was
obtained by applying the specific spot rates along the yield curve used in the determination of the benefit obligation to the relevant projected cash flows.
In May 2023, to further its pension de-risking strategy, the Company settled certain pension obligations in the Netherlands through the purchase of an annuity. A non-cash settlement charge of approximately $27 million was recognized.
In November 2022, to further its pension de-risking strategy the Company purchased an annuity for portions of its U.S. pension plans that will settle certain obligations. This triggered settlement accounting which required immediate recognition of a portion of the accumulated losses associated with the plan. Consequently, the Company recognized a non-cash settlement charge of approximately $170 million in 2022.
Transfer payments from certain U.K. pension plans exceeded the plan’s service and interest cost in 2021. This triggered settlement accounting which required immediate recognition of a portion of the accumulated losses associated with the plan. Consequently, the Company recognized a non-cash settlement charge for approximately £3 million in 2021 ($5 million using December 31, 2021 exchange rates).
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%
Expected Return on Plan Assets
To determine the expected long-term rate of return on plan assets, the historical performance, investment community forecasts, and current market conditions are analyzed to develop expected returns for each asset class used by the plans. The expected returns for each asset class are weighted by the target allocations of the plans. The expected return of 6.82% on U.S. plan assets reflects a portfolio that is seeking asset growth through a higher equity allocation while maintaining prudent risk levels. The portfolio contains certain assets that have historically resulted in higher returns, as well as other financial instruments to minimize downside risk.
No plan assets are expected to be returned to the Company during 2024.
Fair value of plan assets
The Company determined the fair value of plan assets through numerous procedures based on the asset class and available information. Refer to Note 15 “Fair Value Measurements and Financial Instruments” for a description of the procedures performed to determine the fair value of the 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 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 
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.
Investment Policy and Strategy
The U.S. investment policy, as established by the RPGIC, seeks reasonable asset growth at prudent risk levels within weighted average target allocations. At December 31, 2023, the weighted average targeted allocation for the U.S. plans was 28% for equity investments, 50% for fixed income investments, and 22% for other investments. Aon believes that plan assets are well-diversified and are of appropriate quality. The investment portfolio asset allocation is reviewed quarterly and re-balanced to be within policy target allocations. The investment policy is reviewed at least annually and revised, as deemed appropriate by the RPGIC. The investment policies for international plans are generally established by the local pension plan trustees and seek to maintain the plans’ ability to meet liabilities and to comply with local minimum funding requirements. Plan assets are invested in diversified portfolios that provide adequate levels of return at an acceptable level of risk. The investment policies are reviewed at least annually and revised, as deemed appropriate to ensure that the objectives are being met. At December 31, 2023, the weighted average targeted allocation for the U.K. and non-U.S. plans was 6% for equity investments, 89% for fixed income investments, and 5% for other investments.
Cash Flows
Contributions
Based on current assumptions, in 2024, the Company expects to contribute approximately $2 million, $53 million, and $13 million to its significant U.K., U.S., and other major pension plans, respectively.
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 
U.S. and Canadian Other Postretirement Benefits
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)
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%
Based on current assumptions, the Company expects:
•The amount in Accumulated other comprehensive income expected to be recognized as a component of net periodic benefit cost during 2024 is $0.7 million net gain and $0.2 million of prior-service credit.
•To contribute $6 million to fund significant other postretirement benefit plans during 2024.
•Estimated future benefit payments will be approximately $6 million each year for 2024 through 2028, and $28 million in aggregate for 2029-2033.