Corporate Investments and Funds Held For Clients - Available-for-Sale Securities That Have Been in an Unrealized Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2025	Jun. 30, 2024
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (10.2)	$ (66.8)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(672.1)	(1,479.9)
Total, Gross Unrealized Losses	(682.3)	(1,546.7)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,963.9	5,560.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	16,184.9	22,249.9
Total, Fair Market Value	18,148.8	27,809.9
Corporate bonds
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(5.4)	(25.8)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(414.8)	(919.0)
Total, Gross Unrealized Losses	(420.2)	(944.8)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	690.4	2,173.6
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	10,218.8	12,413.4
Total, Fair Market Value	10,909.2	14,587.0
U.S. Treasury securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.8)	(23.1)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(42.9)	(141.4)
Total, Gross Unrealized Losses	(43.7)	(164.5)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	732.2	2,186.2
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,801.7	4,076.9
Total, Fair Market Value	2,533.9	6,263.1
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(1.4)	(0.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(31.0)	(85.7)
Total, Gross Unrealized Losses	(32.4)	(86.6)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	219.2	304.6
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	870.2	1,591.6
Total, Fair Market Value	1,089.4	1,896.2
U.S. government agency securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.1)	(0.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(79.9)	(139.9)
Total, Gross Unrealized Losses	(80.0)	(140.6)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	8.0	51.5
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,093.8	1,428.2
Total, Fair Market Value	1,101.8	1,479.7
Asset-backed securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.2)	(2.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(15.3)	(40.7)
Total, Gross Unrealized Losses	(15.5)	(43.0)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	101.4	351.4
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	481.4	668.0
Total, Fair Market Value	582.8	1,019.4
Canadian provincial bonds
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.8)	(1.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(26.9)	(54.9)
Total, Gross Unrealized Losses	(27.7)	(56.2)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	44.3	193.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	588.8	717.4
Total, Fair Market Value	633.1	910.4
Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses		(0.5)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses		(34.6)
Total, Gross Unrealized Losses		(35.1)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value		11.2
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value		489.6
Total, Fair Market Value		500.8
Other securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(1.5)	(12.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(61.3)	(63.7)
Total, Gross Unrealized Losses	(62.8)	(75.9)
Debt Securities, Available-for-Sale, Unrealized Loss Position [Abstract]
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	168.4	288.5
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,130.2	864.8
Total, Fair Market Value	$ 1,298.6	$ 1,153.3