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, 2024	Jun. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (66.8)	$ (179.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	5,560.0	8,235.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(1,479.9)	(2,034.6)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	22,249.9	20,814.5
Total, Gross Unrealized Losses	(1,546.7)	(2,214.4)
Total, Fair Market Value	27,809.9	29,049.5
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(25.8)	(62.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	2,173.6	2,255.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(919.0)	(1,246.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	12,413.4	12,050.5
Total, Gross Unrealized Losses	(944.8)	(1,308.3)
Total, Fair Market Value	14,587.0	14,306.4
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(23.1)	(85.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	2,186.2	4,629.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(141.4)	(204.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	4,076.9	2,876.3
Total, Gross Unrealized Losses	(164.5)	(290.4)
Total, Fair Market Value	6,263.1	7,505.7
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	(0.9)	(5.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	304.6	333.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(85.7)	(139.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,591.6	1,588.0
Total, Gross Unrealized Losses	(86.6)	(145.0)
Total, Fair Market Value	1,896.2	1,921.9
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.7)	(0.6)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	51.5	28.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(139.9)	(179.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,428.2	1,432.2
Total, Gross Unrealized Losses	(140.6)	(179.8)
Total, Fair Market Value	1,479.7	1,460.4
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(2.3)	(2.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	351.4	159.7
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(40.7)	(67.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	668.0	975.6
Total, Gross Unrealized Losses	(43.0)	(69.7)
Total, Fair Market Value	1,019.4	1,135.3
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(1.3)	(2.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	193.0	127.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(54.9)	(75.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	717.4	757.3
Total, Gross Unrealized Losses	(56.2)	(78.1)
Total, Fair Market Value	910.4	884.3
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.5)	(6.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	11.2	126.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(34.6)	(40.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	489.6	505.6
Total, Gross Unrealized Losses	(35.1)	(46.7)
Total, Fair Market Value	500.8	632.5
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(12.2)	(14.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	288.5	574.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(63.7)	(81.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	864.8	629.0
Total, Gross Unrealized Losses	(75.9)	(96.4)
Total, Fair Market Value	$ 1,153.3	$ 1,203.0