Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 14,341.4	$ 9,250.5
Operating revenues	450.5	380.1
Net operating revenues (loss)	313.8	256.1
Net contribution	240.0	198.6
Segment income	145.3	115.2
Net costs not allocated to operating segments	(92.8)	(88.3)
Other gain	0.0	0.0
Income before tax	52.5	26.9
Assets	19,229.0		$ 18,839.6
Corporate Unallocated
Segment Reporting Information [Line Items]
Revenues	2.1	(3.5)
Operating revenues	2.1	(3.5)
Net operating revenues (loss)	(13.9)	(16.6)
Assets	579.5		841.7
Eliminations
Segment Reporting Information [Line Items]
Revenues	(4.3)	(3.6)
Operating revenues	(4.3)	(3.6)
Commercial
Segment Reporting Information [Line Items]
Net contribution	90.7	56.8
Segment income	65.5	32.1
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	13,823.6	8,865.5
Operating revenues	152.6	105.6
Net operating revenues (loss)	129.7	82.6
Assets	3,641.7		3,969.9
Institutional
Segment Reporting Information [Line Items]
Net contribution	57.4	67.1
Segment income	31.9	44.8
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	161.3	165.5
Operating revenues	161.3	165.5
Net operating revenues (loss)	92.9	105.3
Assets	13,014.1		12,403.3
Retail
Segment Reporting Information [Line Items]
Net contribution	60.0	48.6
Segment income	23.4	17.9
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	316.3	192.2
Operating revenues	96.4	81.7
Net operating revenues (loss)	64.8	52.1
Assets	1,645.0		1,380.9
Global Payments
Segment Reporting Information [Line Items]
Net contribution	31.9	26.1
Segment income	24.5	20.4
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	42.4	34.4
Operating revenues	42.4	34.4
Net operating revenues (loss)	40.3	$ 32.7
Assets	$ 348.7		$ 243.8