Segment Analysis - Segment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 11,244.8	$ 20,366.3	$ 20,495.3	$ 31,611.3
Operating revenues	471.4	366.8	851.5	643.6
Net operating revenues	334.2	243.4	590.3	413.9
Net contribution	248.3	174.9	446.9	300.7
Segment income	159.0	117.3	274.2	193.1
Net costs not allocated to operating segments	(82.7)	(61.2)	(171.0)	(115.4)
Other gain	0.0	0.0	0.0	0.1
Income before tax	76.3	56.1	103.2	77.8
Assets	15,799.7		15,799.7		$ 13,474.9
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Revenues	4.2	4.0	0.7	9.2
Operating revenues	4.2	4.0	0.7	9.2
Net operating revenues	(10.2)	(3.5)	(26.8)	(6.2)
Assets	699.7		699.7		418.7
Eliminations
Segment Reporting Information [Line Items]
Revenues	(4.4)	(6.7)	(8.0)	(13.3)
Operating revenues	(4.4)	(6.7)	(8.0)	(13.3)
Commercial
Segment Reporting Information [Line Items]
Net contribution	81.8	70.4	138.6	121.9
Segment income	55.6	41.7	87.7	70.4
Commercial | Operating Segments
Segment Reporting Information [Line Items]
Revenues	10,516.4	20,037.4	19,381.9	31,043.7
Operating revenues	144.3	126.2	249.9	219.4
Net operating revenues	118.2	104.3	200.8	178.0
Assets	3,438.7		3,438.7		2,753.6
Institutional
Segment Reporting Information [Line Items]
Net contribution	75.3	72.5	142.4	118.5
Segment income	52.0	51.4	96.8	76.7
Institutional | Operating Segments
Segment Reporting Information [Line Items]
Revenues	191.6	185.7	357.1	318.1
Operating revenues	191.6	185.7	357.1	318.1
Net operating revenues	123.8	104.9	229.1	169.7
Assets	9,912.8		9,912.8		8,740.8
Retail
Segment Reporting Information [Line Items]
Net contribution	65.5	9.4	114.1	13.9
Segment income	32.0	7.0	49.9	9.9
Retail | Operating Segments
Segment Reporting Information [Line Items]
Revenues	503.5	116.5	695.7	192.8
Operating revenues	102.2	28.2	183.9	49.4
Net operating revenues	70.7	9.7	122.8	14.6
Assets	1,382.3		1,382.3		1,245.9
Global Payments
Segment Reporting Information [Line Items]
Net contribution	25.7	22.6	51.8	46.4
Segment income	19.4	17.2	39.8	36.1
Global Payments | Operating Segments
Segment Reporting Information [Line Items]
Revenues	33.5	29.4	67.9	60.8
Operating revenues	33.5	29.4	67.9	60.8
Net operating revenues	31.7	$ 28.0	64.4	$ 57.8
Assets	$ 366.2		$ 366.2		$ 315.9