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Segment and Geographic Information - Additional Information (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 USD ($) employee country market | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Sep. 30, 2020 USD ($) employee currency market country | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($)
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of employees | employee | 2,900 |  |  |  |  |  |  |  | 2,900 |  |  |
Number of countries in which entity operates (more than) | country | 180 |  |  |  |  |  |  |  | 180 |  |  |
Total revenues | $ 14,284.9 | $ 8,243.4 | $ 20,366.3 | $ 11,245.0 | $ 11,279.6 | $ 7,873.0 | $ 7,192.2 | $ 6,552.2 | $ 54,139.6 | $ 32,897.0 | $ 27,622.7 |
Operating revenues | 342.1 | 322.6 | 366.8 | 276.8 | 286.9 | 283.4 | 271.1 | 264.7 | 1,308.3 | 1,106.1 | 975.8 |
Net operating revenues (loss): | 226.2 | 227.9 | 243.4 | 170.5 | 173.4 | 165.6 | 165.2 | 149.0 | 868.0 | 653.2 | 581.6 |
Net contribution: |  |  |  |  |  |  |  |  | 639.5 | 482.8 | 432.4 |
Segment income (loss): |  |  |  |  |  |  |  |  | 395.1 | 305.7 | 261.9 |
Net costs not allocated to operating segments |  |  |  |  |  |  |  |  | (270.3) | (200.2) | (162.4) |
Gain on acquisitions and other gains |  |  |  |  |  |  |  |  | 81.9 | 5.5 | 2.0 |
Income before tax | 79.9 | $ 49.0 | $ 56.1 | $ 21.7 | 34.1 | 21.6 | 30.9 | 24.4 | 206.7 | 111.0 | 101.5 |
(Recovery) bad debt on physical coal |  |  |  |  | (10.0) | $ 0.0 | $ 0.0 | $ (2.4) | 0.0 | (12.4) | 1.0 | $ 47.0
Total assets: | 13,474.9 |  |  |  | 9,936.1 |  |  |  | 13,474.9 | 9,936.1 | 7,824.7 |
Corporate, Non-Segment [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 14.6 | 20.8 | 23.9 |
Operating revenues |  |  |  |  |  |  |  |  | 14.6 | 20.8 | 23.9 |
Net operating revenues (loss): |  |  |  |  |  |  |  |  | (24.5) | (10.8) | (0.3) |
Total assets: | 418.7 |  |  |  | 147.9 |  |  |  | 418.7 | 147.9 | 100.5 |
Eliminations [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | (19.3) | (25.1) | (19.5) |
Operating revenues |  |  |  |  |  |  |  |  | (19.3) | (25.1) | (19.5) |
Commercial [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net contribution: |  |  |  |  |  |  |  |  | 242.2 | 224.6 | 210.0 |
Segment income (loss): |  |  |  |  |  |  |  |  | 141.9 | 144.6 | 118.3 |
Commercial [Member] | Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 52,970.1 | 32,125.4 | 27,018.7 |
Operating revenues |  |  |  |  |  |  |  |  | 431.5 | 404.4 | 371.8 |
Net operating revenues (loss): |  |  |  |  |  |  |  |  | 353.4 | 321.2 | 299.4 |
Total assets: | 2,753.6 |  |  |  | 2,386.4 |  |  |  | 2,753.6 | 2,386.4 | 2,349.5 |
Institutional [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net contribution: |  |  |  |  |  |  |  |  | 248.9 | 157.0 | 135.6 |
Segment income (loss): |  |  |  |  |  |  |  |  | 152.9 | 88.6 | 78.2 |
Institutional [Member] | Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 624.1 | 515.0 | 427.1 |
Operating revenues |  |  |  |  |  |  |  |  | 624.1 | 515.0 | 427.1 |
Net operating revenues (loss): |  |  |  |  |  |  |  |  | 363.8 | 220.1 | 176.3 |
Total assets: | $ 8,740.8 |  |  |  | 7,111.2 |  |  |  | $ 8,740.8 | 7,111.2 | 5,168.1 |
Retail [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of countries in which entity operates (more than) | country | 130 |  |  |  |  |  |  |  | 130 |  |  |
Number of global financial markets | market | 15,000 |  |  |  |  |  |  |  | 15,000 |  |  |
Net contribution: |  |  |  |  |  |  |  |  | $ 58.8 | 14.6 | 11.8 |
Segment income (loss): |  |  |  |  |  |  |  |  | 31.7 | 6.4 | 5.6 |
Retail [Member] | Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 432.7 | 148.1 | 73.3 |
Operating revenues |  |  |  |  |  |  |  |  | 140.0 | 78.2 | 73.3 |
Net operating revenues (loss): |  |  |  |  |  |  |  |  | 63.8 | 15.7 | 12.7 |
Total assets: | $ 1,245.9 |  |  |  | 12.4 |  |  |  | $ 1,245.9 | 12.4 | 0.0 |
Global Payments [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of countries in which entity operates (more than) | country | 170 |  |  |  |  |  |  |  | 170 |  |  |
Number of different types of foreign currencies (more than) | currency |  |  |  |  |  |  |  |  | 140 |  |  |
Net contribution: |  |  |  |  |  |  |  |  | $ 89.6 | 86.6 | 75.0 |
Segment income (loss): |  |  |  |  |  |  |  |  | 68.6 | 66.1 | 59.8 |
Global Payments [Member] | Operating Segments [Member] |  |  |  |  |  |  |  |  |  |  |  |
Segment Reporting Information [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenues |  |  |  |  |  |  |  |  | 117.4 | 112.8 | 99.2 |
Operating revenues |  |  |  |  |  |  |  |  | 117.4 | 112.8 | 99.2 |
Net operating revenues (loss): |  |  |  |  |  |  |  |  | 111.5 | 107.0 | 93.5 |
Total assets: | $ 315.9 |  |  |  | $ 278.2 |  |  |  | $ 315.9 | $ 278.2 | $ 206.6 |

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