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	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Supplemental Cash Flow Elements [Abstract]
Restricted fiduciary cash	$ 1,411.8	$ 1,231.9	$ 583.2
Non-restricted fiduciary cash	190.8	151.3	193.8
Total restricted and non-restricted fiduciary cash at the end of the period	1,602.6	1,383.2	777.0
Cash and cash equivalents	700.3	650.0	693.2
Fiduciary cash	1,602.6	1,383.2	777.0
Total cash and cash equivalents inclusive of restricted cash at the end of the period	$ 2,302.9	$ 2,033.2	$ 1,470.2	$ 1,271.9