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Acquisitions and Dispositions (Narrative) (Details) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Apr. 30, 2022 USD ($) | Sep. 30, 2023 USD ($) | Jun. 30, 2023 USD ($) | Mar. 31, 2023 USD ($) | Sep. 30, 2022 USD ($) | Mar. 31, 2022 USD ($) | Sep. 30, 2023 USD ($) acquisition | Sep. 30, 2022 USD ($) | Dec. 31, 2022 acquisition | Nov. 01, 2022 | Oct. 31, 2022
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  |  | $ 711 | $ 227 |  |  |
Cash paid |  |  |  |  |  |  | 682 | 210 |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  |  | 29 | 17 |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  |  | 52 | 116 |  |  |
Contingent consideration from prior year's acquisitions |  |  |  |  |  |  | 135 | 28 |  |  |
Cash and cash equivalents held in fiduciary capacity |  |  |  | $ 20 |  |  | (18) | 138 |  |  |
Deconsolidation of Russian businesses |  |  |  |  |  | $ (39) | 0 | (39) |  |  |
Purchase of non-controlling interests |  |  | $ 139 |  |  |  | 139 | 0 |  |  |
Beassur SARL |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of ownership in equity investment |  |  |  |  |  |  |  |  |  | 70.00% | 35.00%
U.S. Affinity Business |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale | $ 140 |  |  |  |  |  |  |  |  |  |
Gain on disposal | $ 112 |  |  |  |  |  |  |  |  |  |
Certain Businesses Primarily in Brazil |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cash proceeds from sale |  |  |  |  |  |  |  | 7 |  |  |
Prior Fiscal Periods Acquisitions |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Deferred purchase consideration from prior years' acquisitions |  |  |  |  |  |  |  | 116 |  |  |
Contingent consideration from prior year's acquisitions |  |  |  |  |  |  | 176 | 66 |  |  |
Pro forma revenue |  |  |  |  | $ 16 |  |  | 22 |  |  |
Operating income (loss) related to acquisitions |  |  |  |  | $ 1 |  |  | $ 1 |  |  |
Current Fiscal Period Acquisitions |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total consideration |  |  |  |  |  |  | 711 |  |  |  |
Cash paid |  |  |  |  |  |  | 682 |  |  |  |
Estimated fair value of deferred/contingent consideration |  |  |  |  |  |  | 29 |  |  |  |
Pro forma revenue |  | $ 50 |  |  |  |  | 91 |  |  |  |
Operating income (loss) related to acquisitions |  | 5 |  |  |  |  | 14 |  |  |  |
Advance Asset Management |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Acquisition related costs |  | $ 5 |  |  |  |  | 32 |  |  |  |
Small Individual Financial Advisory Business | Operating Segments |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Pre-tax loss on sale |  |  |  |  |  |  | $ 17 |  |  |  |
Risk and Insurance Services |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  |  | 8 |  | 16 |  |
Consulting |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of acquisitions made (in acquisitions) | acquisition |  |  |  |  |  |  | 4 |  | 4 |  |
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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