Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 10,192.1	$ 8,243.4	$ 30,687.4	$ 39,854.7
Operating revenues	431.5	322.6	1,283.0	966.2
Net operating revenues (loss)	298.0	227.9	888.3	641.8
Net contribution	225.2	163.6	672.1	464.3
Segment income	133.2	110.7	407.4	303.8
Net costs not allocated to operating segments	(90.8)	(61.7)	(261.8)	(177.1)
Gain on acquisitions and other gains	3.6	0.0	3.6	0.1
Income before tax	46.0	49.0	149.2	126.8
Assets	16,799.8		16,799.8		$ 13,474.9
Corporate Unallocated
Segment Reporting Information [Line Items]
Revenues	(2.0)	2.3	(1.3)	11.5
Operating revenues	(2.0)	2.3	(1.3)	11.5
Net operating revenues (loss)	(17.1)	(4.5)	(43.9)	(10.7)
Assets	928.8		928.8		418.7
Eliminations
Segment Reporting Information [Line Items]
Revenues	(4.4)	(3.3)	(12.4)	(16.6)
Operating revenues	(4.4)	(3.3)	(12.4)	(16.6)
Commercial
Segment Reporting Information [Line Items]
Net contribution	86.9	59.2	225.5	181.1
Segment income	60.4	37.2	148.1	107.6
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	9,449.3	7,968.1	28,831.2	39,011.8
Operating revenues	152.2	104.3	402.1	323.7
Net operating revenues (loss)	125.0	86.6	325.8	264.6
Assets	3,357.8		3,357.8		2,753.6
Institutional
Segment Reporting Information [Line Items]
Net contribution	69.9	77.3	212.3	195.8
Segment income	46.5	53.8	143.3	130.5
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	173.0	169.8	530.1	487.9
Operating revenues	173.0	169.8	530.1	487.9
Net operating revenues (loss)	111.1	113.1	340.2	282.8
Assets	10,669.6		10,669.6		8,740.8
Retail
Segment Reporting Information [Line Items]
Net contribution	41.9	6.2	156.0	20.1
Segment income	6.0	3.8	55.9	13.7
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	541.2	79.1	1,236.9	271.9
Operating revenues	77.7	22.1	261.6	71.5
Net operating revenues (loss)	45.9	6.7	168.7	21.3
Assets	1,403.2		1,403.2		1,245.9
Global Payments
Segment Reporting Information [Line Items]
Net contribution	26.5	20.9	78.3	67.3
Segment income	20.3	15.9	60.1	52.0
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	35.0	27.4	102.9	88.2
Operating revenues	35.0	27.4	102.9	88.2
Net operating revenues (loss)	33.1	$ 26.0	97.5	$ 83.8
Assets	$ 440.4		$ 440.4		$ 315.9