Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 16,382.7	$ 11,244.8	$ 30,724.1	$ 20,495.3
Operating revenues	544.7	471.4	995.2	851.5
Net operating revenues (loss)	400.3	334.2	714.1	590.3
Net contribution	304.7	248.3	544.7	446.9
Segment income	189.5	159.0	334.8	274.2
Net costs not allocated to operating segments	(102.1)	(82.7)	(194.9)	(171.0)
Income before tax	87.4	76.3	139.9	103.2
Assets	21,195.7		21,195.7		$ 18,839.6
Corporate Unallocated
Segment Reporting Information [Line Items]
Revenues	1.9	4.2	4.0	0.7
Operating revenues	1.9	4.2	4.0	0.7
Net operating revenues (loss)	(14.0)	(10.2)	(27.9)	(26.8)
Assets	571.8		571.8		841.7
Eliminations
Segment Reporting Information [Line Items]
Revenues	(5.1)	(4.4)	(9.4)	(8.0)
Operating revenues	(5.1)	(4.4)	(9.4)	(8.0)
Commercial
Segment Reporting Information [Line Items]
Net contribution	109.6	81.8	200.3	138.6
Segment income	70.1	55.6	135.6	87.7
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	15,792.5	10,516.4	29,616.1	19,381.9
Operating revenues	184.1	144.3	336.7	249.9
Net operating revenues (loss)	155.8	118.2	285.5	200.8
Assets	5,276.3		5,276.3		3,969.9
Institutional
Segment Reporting Information [Line Items]
Net contribution	82.3	75.3	139.7	142.4
Segment income	50.0	52.0	81.9	96.8
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	202.8	191.6	364.1	357.1
Operating revenues	202.8	191.6	364.1	357.1
Net operating revenues (loss)	132.8	123.8	225.7	229.1
Assets	13,467.1		13,467.1		12,403.3
Retail
Segment Reporting Information [Line Items]
Net contribution	80.7	65.5	140.7	114.1
Segment income	45.5	32.0	68.9	49.9
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	349.6	503.5	665.9	695.7
Operating revenues	120.0	102.2	216.4	183.9
Net operating revenues (loss)	86.8	70.7	151.6	122.8
Assets	1,477.4		1,477.4		1,380.9
Global Payments
Segment Reporting Information [Line Items]
Net contribution	32.1	25.7	64.0	51.8
Segment income	23.9	19.4	48.4	39.8
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	41.0	33.5	83.4	67.9
Operating revenues	41.0	33.5	83.4	67.9
Net operating revenues (loss)	38.9	$ 31.7	79.2	$ 64.4
Assets	$ 403.1		$ 403.1		$ 243.8