Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 9,250.5	$ 11,245.0
Operating revenues	380.1	276.8
Net operating revenues	256.1	170.5
Net contribution	198.6	125.8
Segment income	115.2	75.8
Net costs not allocated to operating segments	(88.3)	(54.2)
Other gain	0.0	0.1
Income before tax	26.9	21.7
Assets	13,974.8		$ 13,474.9
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenues	(3.5)	5.2
Operating revenues	(3.5)	5.2
Net operating revenues	(16.6)	(2.7)
Assets	586.0		418.7
Eliminations
Segment Reporting Information [Line Items]
Revenues	(3.6)	(6.6)
Operating revenues	(3.6)	(6.6)
Commercial
Segment Reporting Information [Line Items]
Net contribution	56.8	51.5
Segment income	32.1	28.7
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	8,865.5	11,006.3
Operating revenues	105.6	93.2
Net operating revenues	82.6	73.7
Assets	3,203.8		2,753.6
Institutional
Segment Reporting Information [Line Items]
Net contribution	67.1	46.0
Segment income	44.8	25.3
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	165.5	132.4
Operating revenues	165.5	132.4
Net operating revenues	105.3	64.8
Assets	8,706.1		8,740.8
Retail
Segment Reporting Information [Line Items]
Net contribution	48.6	4.5
Segment income	17.9	2.9
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	192.2	76.3
Operating revenues	81.7	21.2
Net operating revenues	52.1	4.9
Assets	1,242.4		1,245.9
Global Payments
Segment Reporting Information [Line Items]
Net contribution	26.1	23.8
Segment income	20.4	18.9
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	34.4	31.4
Operating revenues	34.4	31.4
Net operating revenues	32.7	$ 29.8
Assets	$ 236.5		$ 315.9