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Revenue from Contracts with Clients - Narrative (Details) | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018
Revenue from Contract with Customer [Abstract] |  |  |
Total revenues (percentage) | 0.90% | 1.40% | 1.70%
Operating revenues (percentage) | 37.20% | 40.90% | 47.40%
Term of risk management consulting contracts | 6 months |  |

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