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Retirement Benefits (Narrative) (Details) $ in Millions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2017 | Nov. 30, 2019 GBP (£) | Dec. 31, 2020 USD ($) shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) plan
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Period to recognize gains or losses on market-related value of assets |  |  | 5 years |  |
Assumed health care cost trend rate for Medicare eligibles and non-Medicare eligibles |  |  | 4.93% |  |
Future assumed health care cost trend rate |  |  | 4.22% |  |
Automatic employer contribution of employee's eligible base pay (as a percent) | 4.00% |  |  |  |
401(k) Plan's assets invested in common stock |  |  | $ 537 | $ 556 |
U.S. |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Expected employer contributions, next fiscal year |  |  | 37 |  |
Cost of defined contribution plans |  |  | $ 145 | 139 | $ 133
U.S. | Pension and Postretirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Assumed health care cost trend rate for Medicare eligibles and non-Medicare eligibles |  |  | 5.80% |  |
Future assumed health care cost trend rate |  |  | 4.50% |  |
Cap on employer share of health care trend (as a percent) |  |  | 5.00% |  |
U.S. | Pension Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Accumulated benefit obligation for plans with accumulated benefit obligations in excess of plan assets |  |  | $ 6,900 | 6,300 |
Aggregate fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets |  |  | 5,100 | 4,700 |
Projected benefit obligation for plans with projected benefit obligations in excess of plan assets |  |  | 6,900 | 6,300 |
Fair value of plan assets for plans with projected benefit obligations in excess of plan assets |  |  | $ 5,100 | 4,700 |
U.S. | Post-retirement Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Number of shares held in defined benefit plan (in shares) | shares |  |  | 2,000,000 |  |
Change in plan assets (as a percent) |  |  | 4.60% |  |
U.S. | Equities and equity alternatives |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 64.00% |  |
Actual asset allocation percentage |  |  | 64.00% |  |
U.S. | Equities and equity alternatives | Minimum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 59.00% |  |
U.S. | Equities and equity alternatives | Maximum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 69.00% |  |
U.S. | Fixed income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 36.00% |  |
Actual asset allocation percentage |  |  | 36.00% |  |
U.S. | Fixed income | Minimum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 31.00% |  |
U.S. | Fixed income | Maximum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 41.00% |  |
Non-U.S. |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Accumulated benefit obligation for plans with accumulated benefit obligations in excess of plan assets |  |  | $ 3,100 | 2,700 |
Aggregate fair value of plan assets for plans with accumulated benefit obligations in excess of plan assets |  |  | 2,500 | 2,200 |
Projected benefit obligation for plans with projected benefit obligations in excess of plan assets |  |  | 3,300 | 3,000 |
Fair value of plan assets for plans with projected benefit obligations in excess of plan assets |  |  | 2,600 | 2,300 |
Cost of defined contribution plans |  |  | 121 | 100 | $ 80
Non-U.S. | Pension Plan Funded Status Guarantee |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Annual deficit contributions, period |  | 7 years |  |  |
Non-U.S. | Pension Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Non-cash settlement charges |  |  | 3 | $ 7 |
Expected employer contributions, next fiscal year |  |  | 87 |  |
United Kingdom | JLT U.K. Plan |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Annual deficit contributions (GBP) |  |  | $ 29 |  |
United Kingdom | Pension Plan Funded Status Guarantee |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Annual deficit contributions (GBP) | £ |  | £ 450,000,000 |  |  |
United Kingdom | Pension Benefits |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Number of plans for Lump Sum payments exceeded settlement thresholds | plan |  |  |  |  | 2
Non-cash settlement charges |  |  |  |  | $ 42
Period between legally-prescribed negotiations between the Company and the plans' trustee |  |  | 3 years |  |
United Kingdom | Equities and equity alternatives |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 32.00% |  |
Actual asset allocation percentage |  |  | 33.00% |  |
United Kingdom | Equities and equity alternatives | Minimum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 29.00% |  |
United Kingdom | Equities and equity alternatives | Maximum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 35.00% |  |
United Kingdom | Fixed income |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 68.00% |  |
Actual asset allocation percentage |  |  | 67.00% |  |
United Kingdom | Fixed income | Minimum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 65.00% |  |
United Kingdom | Fixed income | Maximum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Target asset allocation percentage |  |  | 71.00% |  |
United Kingdom Pension and Postretirement Benefit Plan, Defined Benefit | United Kingdom | Geographic Concentration Risk |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Concentration risk (as a percent) |  |  | 81.00% |  |

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