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, 2023	Jun. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (26.4)	$ (179.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,841.6	8,235.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(1,534.4)	(2,034.6)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	25,056.8	20,814.5
Total, Gross Unrealized Losses	(1,560.8)	(2,214.4)
Total, Fair Market Value	26,898.4	29,049.5
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(3.6)	(62.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	247.4	2,255.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(931.2)	(1,246.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	13,349.4	12,050.5
Total, Gross Unrealized Losses	(934.8)	(1,308.3)
Total, Fair Market Value	13,596.8	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	(13.3)	(85.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,231.1	4,629.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(161.4)	(204.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	5,589.2	2,876.3
Total, Gross Unrealized Losses	(174.7)	(290.4)
Total, Fair Market Value	6,820.3	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	(0.3)	(5.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	17.1	333.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(101.3)	(139.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,772.8	1,588.0
Total, Gross Unrealized Losses	(101.6)	(145.0)
Total, Fair Market Value	1,789.9	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	(0.1)	(0.6)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	18.8	28.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(144.5)	(179.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,477.8	1,432.2
Total, Gross Unrealized Losses	(144.6)	(179.8)
Total, Fair Market Value	1,496.6	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	(0.5)	(2.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	56.7	159.7
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(46.0)	(67.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	765.5	975.6
Total, Gross Unrealized Losses	(46.5)	(69.7)
Total, Fair Market Value	822.2	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	(0.2)	(2.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	32.1	127.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(55.6)	(75.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	764.5	757.3
Total, Gross Unrealized Losses	(55.8)	(78.1)
Total, Fair Market Value	796.6	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	(0.4)	(6.7)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	11.3	126.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(34.3)	(40.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	557.5	505.6
Total, Gross Unrealized Losses	(34.7)	(46.7)
Total, Fair Market Value	568.8	632.5
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(8.0)	(14.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	227.1	574.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(60.1)	(81.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	780.1	629.0
Total, Gross Unrealized Losses	(68.1)	(96.4)
Total, Fair Market Value	$ 1,007.2	$ 1,203.0