Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 18,930.9	$ 10,192.1	$ 49,655.0	$ 30,687.4
Operating revenues	528.8	431.5	1,524.0	1,283.0
Net operating revenues (loss)	374.1	298.0	1,088.2	888.3
Net contribution	281.9	225.2	826.6	672.1
Segment income	171.1	133.2	505.9	407.4
Net costs not allocated to operating segments	(100.2)	(90.8)	(295.1)	(261.8)
Other gain	0.0	3.6	0.0	3.6
Income before tax	70.9	46.0	210.8	149.2
Assets	19,408.8		19,408.8		$ 18,839.6
Corporate Unallocated
Segment Reporting Information [Line Items]
Revenues	2.9	(2.0)	6.9	(1.3)
Operating revenues	2.9	(2.0)	6.9	(1.3)
Net operating revenues (loss)	(13.5)	(17.1)	(41.4)	(43.9)
Assets	556.4		556.4		841.7
Eliminations
Segment Reporting Information [Line Items]
Revenues	(6.2)	(4.4)	(15.6)	(12.4)
Operating revenues	(6.2)	(4.4)	(15.6)	(12.4)
Commercial
Segment Reporting Information [Line Items]
Net contribution	101.9	86.9	302.2	225.5
Segment income	72.5	60.4	208.1	148.1
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	18,368.7	9,449.3	47,984.8	28,831.2
Operating revenues	170.2	152.2	506.9	402.1
Net operating revenues (loss)	141.9	125.0	427.4	325.8
Assets	4,883.5		4,883.5		3,969.9
Institutional
Segment Reporting Information [Line Items]
Net contribution	74.7	69.9	214.4	212.3
Segment income	47.7	46.5	129.6	143.3
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	209.1	173.0	573.2	530.1
Operating revenues	209.1	173.0	573.2	530.1
Net operating revenues (loss)	126.4	111.1	352.1	340.2
Assets	11,867.3		11,867.3		12,403.3
Retail
Segment Reporting Information [Line Items]
Net contribution	71.7	41.9	212.4	156.0
Segment income	26.3	6.0	95.2	55.9
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	312.1	541.2	978.0	1,236.9
Operating revenues	108.5	77.7	324.9	261.6
Net operating revenues (loss)	77.9	45.9	229.5	168.7
Assets	1,558.6		1,558.6		1,380.9
Global Payments
Segment Reporting Information [Line Items]
Net contribution	33.6	26.5	97.6	78.3
Segment income	24.6	20.3	73.0	60.1
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	44.3	35.0	127.7	102.9
Operating revenues	44.3	35.0	127.7	102.9
Net operating revenues (loss)	41.4	$ 33.1	120.6	$ 97.5
Assets	$ 543.0		$ 543.0		$ 243.8