Corporate Investments and Funds Held for Clients - Available-For-Sale Securities that Have Been in an Unrealized Loss Position (Details) - USD ($) $ in Millions	Mar. 31, 2025	Jun. 30, 2024
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (29.1)	$ (66.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	3,389.3	5,560.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(827.1)	(1,479.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	18,503.8	22,249.9
Total, Gross Unrealized Losses	(856.2)	(1,546.7)
Total, Fair Market Value	21,893.1	27,809.9
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(18.9)	(25.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,724.1	2,173.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(517.8)	(919.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	11,020.7	12,413.4
Total, Gross Unrealized Losses	(536.7)	(944.8)
Total, Fair Market Value	12,744.8	14,587.0
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(4.0)	(23.1)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,004.4	2,186.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(58.5)	(141.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	2,780.4	4,076.9
Total, Gross Unrealized Losses	(62.5)	(164.5)
Total, Fair Market Value	3,784.8	6,263.1
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.1)	(0.9)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	19.9	304.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(36.9)	(85.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,115.6	1,591.6
Total, Gross Unrealized Losses	(37.0)	(86.6)
Total, Fair Market Value	1,135.5	1,896.2
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.1)	(0.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	22.9	51.5
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(91.7)	(139.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,304.1	1,428.2
Total, Gross Unrealized Losses	(91.8)	(140.6)
Total, Fair Market Value	1,327.0	1,479.7
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(1.3)	(2.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	230.8	351.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(19.8)	(40.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	530.9	668.0
Total, Gross Unrealized Losses	(21.1)	(43.0)
Total, Fair Market Value	761.7	1,019.4
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(1.4)	(1.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	68.6	193.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(30.2)	(54.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	599.1	717.4
Total, Gross Unrealized Losses	(31.6)	(56.2)
Total, Fair Market Value	667.7	910.4
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.1)	(0.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	4.0	11.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(21.9)	(34.6)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	419.5	489.6
Total, Gross Unrealized Losses	(22.0)	(35.1)
Total, Fair Market Value	423.5	500.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(3.2)	(12.2)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	314.6	288.5
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(50.3)	(63.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	733.5	864.8
Total, Gross Unrealized Losses	(53.5)	(75.9)
Total, Fair Market Value	$ 1,048.1	$ 1,153.3