Basis of Presentation and Consolidation and Accounting Standards Adopted (Details) $ in Millions		3 Months Ended
	Oct. 01, 2020 USD ($)	Dec. 31, 2020 USD ($) country client	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of commercial and institutional clients (more than) | client		32,000
Number retail clients (more than) | client		330,000
Number of countries in which entity operates (more than) | country		180
Selling and marketing		$ 8.8	$ 1.4
Cumulative-effect adjustment, net of tax		585.7		$ 572.4
Income tax expense		$ 7.4	$ 5.4
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 (more than) | country		130