Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 15,047.9	$ 18,930.9	$ 44,220.5	$ 49,655.0
Operating revenues	776.9	528.8	2,136.1	1,524.0
Net operating revenues (loss)	435.9	374.1	1,217.3	1,088.2
Net contribution	340.8	281.9	949.1	826.6
Segment income	207.9	171.1	560.2	505.9
Net costs not allocated to operating segments	(113.4)	(100.2)	(336.1)	(295.1)
Gain on acquisition	0.0	0.0	23.5	0.0
Income before tax	94.5	70.9	247.6	210.8
Assets	21,933.1		21,933.1		$ 19,859.6
Corporate Unallocated
Segment Reporting Information [Line Items]
Revenues	8.6	2.9	23.9	6.9
Operating revenues	8.6	2.9	23.9	6.9
Net operating revenues (loss)	(14.1)	(13.5)	(47.0)	(41.4)
Assets	725.1		725.1		746.3
Eliminations
Segment Reporting Information [Line Items]
Revenues	(10.2)	(6.2)	(29.1)	(15.6)
Operating revenues	(10.2)	(6.2)	(29.1)	(15.6)
Commercial
Segment Reporting Information [Line Items]
Net contribution	156.9	101.9	413.5	302.2
Segment income	117.0	72.5	302.7	208.1
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	14,444.5	18,368.7	42,188.7	47,984.8
Operating revenues	252.7	170.2	655.2	506.9
Net operating revenues (loss)	213.6	141.9	551.4	427.4
Assets	5,036.6		5,036.6		5,931.0
Institutional
Segment Reporting Information [Line Items]
Net contribution	81.7	74.7	265.6	214.4
Segment income	45.1	47.7	162.9	129.6
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	381.1	209.1	1,087.1	573.2
Operating revenues	381.1	209.1	1,087.1	573.2
Net operating revenues (loss)	120.3	126.4	401.4	352.1
Assets	14,594.1		14,594.1		11,687.1
Retail
Segment Reporting Information [Line Items]
Net contribution	60.3	71.7	147.9	212.4
Segment income	17.2	26.3	17.8	95.2
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	170.7	312.1	791.5	978.0
Operating revenues	91.5	108.5	240.6	324.9
Net operating revenues (loss)	65.1	77.9	159.8	229.5
Assets	1,022.2		1,022.2		971.2
Global Payments
Segment Reporting Information [Line Items]
Net contribution	41.9	33.6	122.1	97.6
Segment income	28.6	24.6	76.8	73.0
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	53.2	44.3	158.4	127.7
Operating revenues	53.2	44.3	158.4	127.7
Net operating revenues (loss)	51.0	$ 41.4	151.7	$ 120.6
Assets	$ 555.1		$ 555.1		$ 524.0