Corporate Investments and Funds Held for Clients - Schedule of Available-For-Sale Securities that Have Been in an Unrealized Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (7.2)	$ (10.2)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,801.0	1,963.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(537.6)	(672.1)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	14,786.6	16,184.9
Total, Gross Unrealized Losses	(544.8)	(682.3)
Total, Fair Market Value	16,587.6	18,148.8
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(1.4)	(5.4)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	292.7	690.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(332.3)	(414.8)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	9,439.8	10,218.8
Total, Gross Unrealized Losses	(333.7)	(420.2)
Total, Fair Market Value	9,732.5	10,909.2
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(4.3)	(0.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	955.1	732.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(28.0)	(42.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,809.1	1,801.7
Total, Gross Unrealized Losses	(32.3)	(43.7)
Total, Fair Market Value	2,764.2	2,533.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.2)	(0.2)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	74.7	101.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(10.9)	(15.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	469.7	481.4
Total, Gross Unrealized Losses	(11.1)	(15.5)
Total, Fair Market Value	544.4	582.8
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)	(1.4)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	175.1	219.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(24.7)	(31.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	686.4	870.2
Total, Gross Unrealized Losses	(25.0)	(32.4)
Total, Fair Market Value	861.5	1,089.4
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.3)	(0.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	55.8	44.3
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(22.3)	(26.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	547.5	588.8
Total, Gross Unrealized Losses	(22.6)	(27.7)
Total, Fair Market Value	603.3	633.1
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)	(0.1)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	10.4	8.0
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(68.9)	(79.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	773.1	1,093.8
Total, Gross Unrealized Losses	(69.0)	(80.0)
Total, Fair Market Value	783.5	1,101.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.6)	(1.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	237.2	168.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(50.5)	(61.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,061.0	1,130.2
Total, Gross Unrealized Losses	(51.1)	(62.8)
Total, Fair Market Value	$ 1,298.2	$ 1,298.6