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, 2024	Jun. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (57.0)	$ (179.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	4,792.1	8,235.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(1,557.2)	(2,034.6)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	23,243.8	20,814.5
Total, Gross Unrealized Losses	(1,614.2)	(2,214.4)
Total, Fair Market Value	28,035.9	29,049.5
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(17.4)	(62.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,543.5	2,255.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(957.8)	(1,246.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	12,649.1	12,050.5
Total, Gross Unrealized Losses	(975.2)	(1,308.3)
Total, Fair Market Value	14,192.6	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	(25.0)	(85.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	2,341.0	4,629.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(153.4)	(204.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	4,651.5	2,876.3
Total, Gross Unrealized Losses	(178.4)	(290.4)
Total, Fair Market Value	6,992.5	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	(1.2)	(5.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	209.3	333.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(97.9)	(139.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,746.1	1,588.0
Total, Gross Unrealized Losses	(99.1)	(145.0)
Total, Fair Market Value	1,955.4	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	(0.2)	(0.6)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	38.5	28.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(145.2)	(179.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,432.3	1,432.2
Total, Gross Unrealized Losses	(145.4)	(179.8)
Total, Fair Market Value	1,470.8	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	(1.6)	(2.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	220.5	159.7
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(44.2)	(67.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	706.6	975.6
Total, Gross Unrealized Losses	(45.8)	(69.7)
Total, Fair Market Value	927.1	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	(1.0)	(2.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	122.4	127.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(58.4)	(75.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	750.0	757.3
Total, Gross Unrealized Losses	(59.4)	(78.1)
Total, Fair Market Value	872.4	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	(35.5)	(40.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	526.8	505.6
Total, Gross Unrealized Losses	(36.0)	(46.7)
Total, Fair Market Value	538.0	632.5
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(10.1)	(14.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	305.7	574.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(64.8)	(81.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	781.4	629.0
Total, Gross Unrealized Losses	(74.9)	(96.4)
Total, Fair Market Value	$ 1,087.1	$ 1,203.0