Basis of Presentation and Consolidation and Accounting Standards Adopted (Details) $ in Millions		3 Months Ended		6 Months Ended
	Oct. 01, 2020 USD ($)	Mar. 31, 2022 USD ($) client country	Mar. 31, 2021 USD ($)	Mar. 31, 2022 USD ($) client country	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of countries in which entity operates | country		180		180
Number of commercial and institutional clients | client		52,000		52,000
Number retail clients | client		370,000		370,000
Cumulative-effect adjustment, net of tax		$ 788.2		$ 788.2		$ 682.5
Income tax expense		$ 23.4	$ 21.0	$ 34.2	$ 28.4
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 | country		180		180