Corporate Investments and Funds Held For Clients - Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Millions		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Corporate Investments and Funds Held for Clients [Line Items]
Cash and cash equivalents, amortized cost		$ 402.0	$ 294.0
Funds held for clients cash and cash equivalents, amortized cost		3,640.8	2,128.8
Total investments, amortized cost		4,067.7	2,622.6
Gross unrealized losses		(0.2)	(1.2)
Cash and cash equivalents, fair value		402.0	294.0	$ 400.7
Funds held for clients cash and cash equivalents, fair value		3,640.8	2,128.8
Total investments, fair value		4,067.5	2,621.4
Available-for-sale Securities [Member] | U.S. Treasury Securities [Member]
Corporate Investments and Funds Held for Clients [Line Items]
Amortized cost	[1]	24.9	174.8
Gross unrealized losses	[1]	(0.2)	(1.2)
Fair value	[1]	$ 24.7	173.6
Available-for-sale Securities [Member] | Certificates of Deposit [Member]
Corporate Investments and Funds Held for Clients [Line Items]
Amortized cost	[1]		25.0
Fair value	[1]		$ 25.0

[1] All available-for-sale securities were included within the funds held for clients.