Corporate Investments and Funds Held For Clients - Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Millions		Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024
Corporate Investments and Funds Held for Clients [Line Items]
Cash and cash equivalents, amortized cost		$ 532.2	$ 402.0
Funds held for clients cash and cash equivalents, amortized cost		1,120.9	3,640.8
Total investments, amortized cost		2,148.8	4,067.7
Gross unrealized gains		0.4
Gross unrealized losses		(0.1)	(0.2)
Cash and cash equivalents, fair value		532.2	402.0	$ 346.5
Funds held for clients cash and cash equivalents, fair value		1,120.9	3,640.8
Total investments, fair value		2,149.1	4,067.5
Available-for-sale Securities [Member] | U.S. Treasury Securities [Member]
Corporate Investments and Funds Held for Clients [Line Items]
Amortized cost	[1]	495.7	24.9
Gross unrealized gains	[1]	0.4
Gross unrealized losses	[1]	(0.1)	(0.2)
Fair value	[1]	$ 496.0	$ 24.7

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