Corporate Investments and Funds Held for Clients - Available-For-Sale Securities that Have Been in an Unrealized Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (14.8)	$ (66.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	658.4	5,560.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(930.0)	(1,479.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	20,799.0	22,249.9
Total, Gross Unrealized Losses	(944.8)	(1,546.7)
Total, Fair Market Value	21,457.4	27,809.9
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(2.4)	(25.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	346.9	2,173.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(576.2)	(919.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	12,170.6	12,413.4
Total, Gross Unrealized Losses	(578.6)	(944.8)
Total, Fair Market Value	12,517.5	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	(5.5)	(23.1)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	77.9	2,186.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(76.2)	(141.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	3,172.5	4,076.9
Total, Gross Unrealized Losses	(81.7)	(164.5)
Total, Fair Market Value	3,250.4	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	0.0	(0.9)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1.4	304.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(52.2)	(85.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,607.2	1,591.6
Total, Gross Unrealized Losses	(52.2)	(86.6)
Total, Fair Market Value	1,608.6	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	0.0	(0.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	0.8	51.5
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(98.2)	(139.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,384.3	1,428.2
Total, Gross Unrealized Losses	(98.2)	(140.6)
Total, Fair Market Value	1,385.1	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	(0.4)	(2.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	96.1	351.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(25.2)	(40.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	624.0	668.0
Total, Gross Unrealized Losses	(25.6)	(43.0)
Total, Fair Market Value	720.1	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	0.0	(1.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1.1	193.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(35.6)	(54.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	661.2	717.4
Total, Gross Unrealized Losses	(35.6)	(56.2)
Total, Fair Market Value	662.3	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	(0.3)	(0.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	15.3	11.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(21.4)	(34.6)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	460.1	489.6
Total, Gross Unrealized Losses	(21.7)	(35.1)
Total, Fair Market Value	475.4	500.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(6.2)	(12.2)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	118.9	288.5
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(45.0)	(63.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	719.1	864.8
Total, Gross Unrealized Losses	(51.2)	(75.9)
Total, Fair Market Value	$ 838.0	$ 1,153.3