Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 13,011.6	$ 14,341.4
Operating revenues	654.8	450.5
Net operating revenues (loss)	382.0	313.8
Net contribution	291.6	240.0
Segment income	172.9	145.3
Net costs not allocated to operating segments	(100.8)	(92.8)
Gain on acquisition	23.5	0.0
Income before tax	95.6	52.5
Assets	19,832.4		$ 19,859.6
Corporate Unallocated
Segment Reporting Information [Line Items]
Revenues	12.8	2.1
Operating revenues	12.8	2.1
Net operating revenues (loss)	(11.1)	(13.9)
Assets	750.3		746.3
Eliminations
Segment Reporting Information [Line Items]
Revenues	(9.8)	(4.3)
Operating revenues	(9.8)	(4.3)
Commercial
Segment Reporting Information [Line Items]
Net contribution	115.7	90.7
Segment income	82.8	65.5
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	12,293.5	13,823.6
Operating revenues	182.4	152.6
Net operating revenues (loss)	152.7	129.7
Assets	5,225.8		5,931.0
Institutional
Segment Reporting Information [Line Items]
Net contribution	94.6	57.4
Segment income	62.0	31.9
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	343.5	161.3
Operating revenues	343.5	161.3
Net operating revenues (loss)	143.2	92.9
Assets	12,482.4		11,687.1
Retail
Segment Reporting Information [Line Items]
Net contribution	39.2	60.0
Segment income	(4.2)	23.4
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	316.2	316.3
Operating revenues	70.5	96.4
Net operating revenues (loss)	43.9	64.8
Assets	953.5		971.2
Global Payments
Segment Reporting Information [Line Items]
Net contribution	42.1	31.9
Segment income	32.3	24.5
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	55.4	42.4
Operating revenues	55.4	42.4
Net operating revenues (loss)	53.3	$ 40.3
Assets	$ 420.4		$ 524.0