Supplemental Disclosures of Cash Flow Information and Non-Cash Financing and Investing Activities - Schedule of Reconciliation of Cash and Cash Equivalents Inclusive of Restricted Cash (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Supplemental Cash Flow Elements [Abstract]
Restricted fiduciary cash	$ 1,325.9	$ 1,231.9
Non-restricted fiduciary cash	189.3	151.3
Total restricted and non-restricted fiduciary cash at the end of the period	1,515.2	1,383.2
Cash and cash equivalents	755.7	650.0	$ 579.5
Fiduciary cash	1,515.2		1,271.3
Total cash, cash equivalents and fiduciary cash at the end of the period	$ 2,270.9	$ 2,033.2	$ 1,850.8	$ 1,470.2