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