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Acquisitions and Dispositions (Narrative) (Details) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2024 USD ($) | Mar. 31, 2024 USD ($) acquisition | Jun. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | Dec. 31, 2023 acquisition
Business Acquisition [Line Items] |  |  |  |  |
Total consideration |  | $ 359 |  | $ 15 |
Cash paid |  | 308 |  | 11 |
Estimated fair value of deferred/contingent purchase consideration |  | 51 |  | 4 |
Deferred purchase consideration from prior years' acquisitions |  | 3 |  | 12 |
Contingent consideration from prior year's acquisitions |  | 12 |  | 1 |
Cash divested from deconsolidation |  |  |  | 20 |
Purchase of non-controlling interests |  |  | $ 139 |  |
Total Segments | Small Individual Financial Advisory Business |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Pre-tax loss on sale |  |  |  | 19 |
Net gain includes in revenue consolidated statements of income |  |  |  | (19) |
Small Individual Financial Advisory Business | Total Segments | Discontinued Operations, Disposed of by Sale |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Pre-tax loss on sale |  | (21) |  |  |
Net gain includes in revenue consolidated statements of income |  | 21 |  |  |
U.S. Health And Benefits And U.K Pension Administration Businesses | Discontinued Operations, Disposed of by Sale |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Cash proceeds from sale | $ 114 |  |  |  |
Pre-tax loss on sale | (21) |  |  |  |
Net gain includes in revenue consolidated statements of income | 21 |  |  |  |
Exit costs | $ 18 |  |  |  |
Current Fiscal Period Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Total consideration |  | 359 |  |  |
Cash paid |  | 308 |  |  |
Estimated fair value of deferred/contingent purchase consideration |  | 51 |  |  |
Pro forma revenue |  | 14 |  |  |
Operating loss related to acquisitions |  | 6 |  |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  | 12 |
Contingent consideration from prior year's acquisitions |  | 26 |  | 1 |
Westpac Transaction |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquisition related costs |  | $ 3 |  | $ 17 |
Risk and Insurance Services |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  | 2 |  |  | 9
Consulting |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  | 4 |  |  | 5
Minimum |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Revenue target period (in years) |  | 2 years |  | 2 years |
Maximum |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Revenue target period (in years) |  | 4 years |  | 4 years |

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