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, 2023	Jun. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (112.7)	$ (179.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	4,385.8	8,235.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(2,243.1)	(2,034.6)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	25,126.6	20,814.5
Total, Gross Unrealized Losses	(2,355.8)	(2,214.4)
Total, Fair Market Value	29,512.4	29,049.5
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(50.3)	(62.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,297.5	2,255.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(1,357.0)	(1,246.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	13,097.5	12,050.5
Total, Gross Unrealized Losses	(1,407.3)	(1,308.3)
Total, Fair Market Value	14,395.0	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	(33.8)	(85.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	2,090.5	4,629.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(250.0)	(204.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	5,836.0	2,876.3
Total, Gross Unrealized Losses	(283.8)	(290.4)
Total, Fair Market Value	7,926.5	7,505.7
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(3.2)	(2.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	251.0	159.7
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(64.9)	(67.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	801.5	975.6
Total, Gross Unrealized Losses	(68.1)	(69.7)
Total, Fair Market Value	1,052.5	1,135.3
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	(4.3)	(5.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	186.8	333.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(144.9)	(139.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,713.8	1,588.0
Total, Gross Unrealized Losses	(149.2)	(145.0)
Total, Fair Market Value	1,900.6	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	(1.9)	(0.6)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	36.0	28.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(196.0)	(179.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,435.6	1,432.2
Total, Gross Unrealized Losses	(197.9)	(179.8)
Total, Fair Market Value	1,471.6	1,460.4
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(6.9)	(2.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	143.3	127.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(81.1)	(75.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	781.8	757.3
Total, Gross Unrealized Losses	(88.0)	(78.1)
Total, Fair Market Value	925.1	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	0.0	(6.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	0.0	126.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(51.9)	(40.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	609.8	505.6
Total, Gross Unrealized Losses	(51.9)	(46.7)
Total, Fair Market Value	609.8	632.5
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(12.3)	(14.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	380.7	574.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(97.3)	(81.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	850.6	629.0
Total, Gross Unrealized Losses	(109.6)	(96.4)
Total, Fair Market Value	$ 1,231.3	$ 1,203.0