Segment and Geographic Information - Additional Information (Details) $ in Millions	12 Months Ended
	Sep. 30, 2021 USD ($) employee market currency country	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of employees | employee	3,200
Number of countries in which entity operates (more than) | country	180
Total revenues	$ 42,534.2	$ 54,139.6	$ 32,897.0
Operating revenues:	1,673.1	1,308.3	1,106.1
Net operating revenues (loss):	1,150.0	868.0	653.2
Net contribution:	868.7	639.5	482.8
Segment income (loss):	506.2	395.1	305.7
Net costs not allocated to operating segments	(355.5)	(270.3)	(200.2)
Gain on acquisitions and other gains	3.4	81.9	5.5
Income from operations, before tax	154.1	206.7	111.0
Recovery of bad debt on physical coal	0.0	0.0	(12.4)
Total assets:	18,839.6	13,474.9	9,936.1
Corporate Unallocated
Segment Reporting Information [Line Items]
Total revenues	1.7	14.6	20.8
Operating revenues:	1.7	14.6	20.8
Net operating revenues (loss):	(54.8)	(24.5)	(10.8)
Total assets:	841.7	418.7	147.9
Eliminations
Segment Reporting Information [Line Items]
Total revenues	(17.1)	(19.3)	(25.1)
Operating revenues:	(17.1)	(19.3)	(25.1)
Commercial
Segment Reporting Information [Line Items]
Net contribution:	299.7	242.2	224.6
Segment income (loss):	192.2	141.9	144.6
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	39,884.0	52,970.1	32,125.4
Operating revenues:	534.8	431.5	404.4
Net operating revenues (loss):	433.1	353.4	321.2
Total assets:	3,969.9	2,753.6	2,386.4
Institutional
Segment Reporting Information [Line Items]
Net contribution:	260.9	248.9	157.0
Segment income (loss):	167.7	152.9	88.6
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	668.4	624.1	515.0
Operating revenues:	668.4	624.1	515.0
Net operating revenues (loss):	419.4	363.8	220.1
Total assets:	$ 12,403.3	8,740.8	7,111.2
Retail
Segment Reporting Information [Line Items]
Number of countries in which entity operates (more than) | country	180
Number of global financial markets | market	15,000
Net contribution:	$ 204.4	58.8	14.6
Segment income (loss):	67.8	31.7	6.4
Retail | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	1,859.9	432.7	148.1
Operating revenues:	348.0	140.0	78.2
Net operating revenues (loss):	222.4	63.8	15.7
Total assets:	$ 1,380.9	1,245.9	12.4
Global Payments
Segment Reporting Information [Line Items]
Number of countries in which entity operates (more than) | country	185
Number of different types of foreign currencies (more than) | currency	140
Net contribution:	$ 103.7	89.6	86.6
Segment income (loss):	78.5	68.6	66.1
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Total revenues	137.3	117.4	112.8
Operating revenues:	137.3	117.4	112.8
Net operating revenues (loss):	129.9	111.5	107.0
Total assets:	$ 243.8	$ 315.9	$ 278.2