Basis of Presentation and Consolidation and Accounting Standards Adopted (Details) $ in Millions		3 Months Ended		6 Months Ended
	Oct. 01, 2020 USD ($)	Mar. 31, 2021 USD ($) country client	Mar. 31, 2020 USD ($)	Mar. 31, 2021 USD ($) country client	Mar. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of commercial and institutional clients (more than) | client		35,000		35,000
Number retail clients (more than) | client		340,000		340,000
Number of countries in which entity operates (more than) | country		180		180
Selling and marketing		$ 6.5	$ 4.2	$ 15.3	$ 5.6
Cumulative-effect adjustment, net of tax		641.0		641.0		$ 572.4
Income tax expense		$ 21.0	$ 16.8	$ 28.4	$ 22.2
Cumulative Effect, Period of Adoption, Adjustment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Cumulative-effect adjustment, net of tax	$ (6.2)
Income tax expense	$ 2.0
Retail Segment
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of countries in which entity operates (more than) | country		130		130