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Revenue from Contracts with Clients - Narrative (Details) | 3 Months Ended | 6 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020 | Mar. 31, 2021 | Mar. 31, 2020
Revenues from Contracts With Clients [Abstract] |  |  |  |
Total revenues (percentage) | 1.40% | 0.70% | 1.40% | 0.80%
Operating revenues (percentage) | 32.20% | 37.90% | 34.60% | 38.50%

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