Corporate Investments and Funds Held for Clients - Available-For-Sale Securities that Have Been in an Unrealized Loss Position (Details) - USD ($) $ in Millions	Dec. 31, 2024	Jun. 30, 2024
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (85.9)	$ (66.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	7,236.0	5,560.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(1,054.7)	(1,479.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	19,578.3	22,249.9
Total, Gross Unrealized Losses	(1,140.6)	(1,546.7)
Total, Fair Market Value	26,814.3	27,809.9
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(51.8)	(25.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	3,389.6	2,173.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(656.9)	(919.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	11,809.9	12,413.4
Total, Gross Unrealized Losses	(708.7)	(944.8)
Total, Fair Market Value	15,199.5	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	(20.3)	(23.1)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	2,677.3	2,186.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(78.7)	(141.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	2,754.4	4,076.9
Total, Gross Unrealized Losses	(99.0)	(164.5)
Total, Fair Market Value	5,431.7	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	(0.7)	(0.9)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	266.8	304.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(51.0)	(85.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,311.2	1,591.6
Total, Gross Unrealized Losses	(51.7)	(86.6)
Total, Fair Market Value	1,578.0	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	(1.9)	(0.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	113.0	51.5
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(116.2)	(139.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,346.8	1,428.2
Total, Gross Unrealized Losses	(118.1)	(140.6)
Total, Fair Market Value	1,459.8	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	(3.0)	(2.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	271.4	351.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(25.3)	(40.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	601.4	668.0
Total, Gross Unrealized Losses	(28.3)	(43.0)
Total, Fair Market Value	872.8	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	(3.9)	(1.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	166.5	193.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(38.7)	(54.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	612.7	717.4
Total, Gross Unrealized Losses	(42.6)	(56.2)
Total, Fair Market Value	779.2	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	(0.4)	(0.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	25.5	11.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(28.0)	(34.6)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	427.6	489.6
Total, Gross Unrealized Losses	(28.4)	(35.1)
Total, Fair Market Value	453.1	500.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(3.9)	(12.2)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	325.9	288.5
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(59.9)	(63.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	714.3	864.8
Total, Gross Unrealized Losses	(63.8)	(75.9)
Total, Fair Market Value	$ 1,040.2	$ 1,153.3