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Acquisitions and Dispositions (Narrative) (Details) $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 01, 2024 USD ($) | Jun. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) acquisition | Jun. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | Jun. 30, 2024 USD ($) acquisition | Jun. 30, 2023 USD ($) | Dec. 31, 2023 acquisition
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  | $ 716 | $ 340 |
Cash paid |  |  |  |  |  | 662 | 332 |
Estimated fair value of deferred/contingent purchase consideration |  |  |  |  |  | 54 | 8 |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  | 10 | 51 |
Contingent consideration from prior year's acquisitions |  |  |  |  |  | 71 | 134 |
Cash divested from deconsolidation |  |  |  |  | $ 20 |  |  |
Purchase of non-controlling interests |  |  |  | $ 139 |  | 0 | 139 |
Total Segments | Small Individual Financial Advisory Business |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Net gain (loss) on disposition of business |  |  |  |  |  |  | (17) |
Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Cash proceeds from sale | $ 114 |  |  |  |  |  |  |
Net gain (loss) on disposition of business | $ 21 |  |  |  |  |  |  |
Exit costs |  |  | $ 18 |  |  |  |  |
Discontinued Operations, Disposed of by Sale | U.S. Health And Benefits And U.K Pension Administration Businesses | Total Segments |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Net gain (loss) on disposition of business |  |  |  |  |  | 21 |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  |  | 51 |
Contingent consideration from prior year's acquisitions |  |  |  |  |  | 161 | 175 |
Pro forma revenue |  |  |  | 41 |  |  | 41 |
Operating income (loss) related to acquisitions |  |  |  | 9 |  |  | 9 |
Current Fiscal Period Acquisitions |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  | 716 |  |
Cash paid |  |  |  |  |  | 662 |  |
Estimated fair value of deferred/contingent purchase consideration |  |  |  |  |  | 54 |  |
Pro forma revenue |  | $ 48 |  |  |  | 62 |  |
Operating income (loss) related to acquisitions |  | 3 |  |  |  | (3) |  |
Westpac Transaction |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Acquisition related costs |  | $ 21 |  | $ 10 |  | $ 29 | $ 27 |
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 |
Risk and Insurance Services |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  | 5 |  | 9
Consulting |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  | 5 |  |  | 4 |  |

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