Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenues	$ 16,161.0	$ 16,382.7	$ 29,172.6	$ 30,724.1
Operating revenues	704.4	544.7	1,359.2	995.2
Net operating revenues (loss)	399.4	400.3	781.4	714.1
Net contribution	316.7	304.7	608.3	544.7
Segment income	179.4	189.5	352.3	334.8
Net costs not allocated to operating segments	(121.9)	(102.1)	(222.7)	(194.9)
Gain on acquisition	0.0	0.0	23.5	0.0
Income before tax	57.5	87.4	153.1	139.9
Assets	21,918.9		21,918.9		$ 19,859.6
Corporate Unallocated
Segment Reporting Information [Line Items]
Revenues	2.5	1.9	15.3	4.0
Operating revenues	2.5	1.9	15.3	4.0
Net operating revenues (loss)	(21.8)	(14.0)	(32.9)	(27.9)
Assets	733.4		733.4		746.3
Eliminations
Segment Reporting Information [Line Items]
Revenues	(9.1)	(5.1)	(18.9)	(9.4)
Operating revenues	(9.1)	(5.1)	(18.9)	(9.4)
Commercial
Segment Reporting Information [Line Items]
Net contribution	140.9	109.6	256.6	200.3
Segment income	102.9	70.1	185.7	135.6
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	15,450.7	15,792.5	27,744.2	29,616.1
Operating revenues	220.1	184.1	402.5	336.7
Net operating revenues (loss)	185.1	155.8	337.8	285.5
Assets	4,803.8		4,803.8		5,931.0
Institutional
Segment Reporting Information [Line Items]
Net contribution	89.3	82.3	183.9	139.7
Segment income	55.8	50.0	117.8	81.9
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	362.5	202.8	706.0	364.1
Operating revenues	362.5	202.8	706.0	364.1
Net operating revenues (loss)	137.9	132.8	281.1	225.7
Assets	14,769.5		14,769.5		11,687.1
Retail
Segment Reporting Information [Line Items]
Net contribution	48.4	80.7	87.6	140.7
Segment income	4.8	45.5	0.6	68.9
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	304.6	349.6	620.8	665.9
Operating revenues	78.6	120.0	149.1	216.4
Net operating revenues (loss)	50.8	86.8	94.7	151.6
Assets	1,128.6		1,128.6		971.2
Global Payments
Segment Reporting Information [Line Items]
Net contribution	38.1	32.1	80.2	64.0
Segment income	15.9	23.9	48.2	48.4
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	49.8	41.0	105.2	83.4
Operating revenues	49.8	41.0	105.2	83.4
Net operating revenues (loss)	47.4	$ 38.9	100.7	$ 79.2
Assets	$ 483.6		$ 483.6		$ 524.0