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	Dec. 31, 2025	Jun. 30, 2025
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (6.8)	$ (10.2)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,924.8	1,963.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(446.3)	(672.1)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	13,323.8	16,184.9
Total, Gross Unrealized Losses	(453.1)	(682.3)
Total, Fair Market Value	15,248.6	18,148.8
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(2.5)	(5.4)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	861.0	690.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(273.7)	(414.8)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	8,816.4	10,218.8
Total, Gross Unrealized Losses	(276.2)	(420.2)
Total, Fair Market Value	9,677.4	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	(2.0)	(0.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	711.9	732.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(15.7)	(42.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,264.6	1,801.7
Total, Gross Unrealized Losses	(17.7)	(43.7)
Total, Fair Market Value	1,976.5	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	58.7	101.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(8.2)	(15.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	392.7	481.4
Total, Gross Unrealized Losses	(8.4)	(15.5)
Total, Fair Market Value	451.4	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	(1.8)	(1.4)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	198.2	219.2
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(22.1)	(31.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	657.2	870.2
Total, Gross Unrealized Losses	(23.9)	(32.4)
Total, Fair Market Value	855.4	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	(0.1)	(0.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	55.5	44.3
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(20.3)	(26.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	535.0	588.8
Total, Gross Unrealized Losses	(20.4)	(27.7)
Total, Fair Market Value	590.5	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	(63.5)	(79.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	666.7	1,093.8
Total, Gross Unrealized Losses	(63.6)	(80.0)
Total, Fair Market Value	677.1	1,101.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.1)	(1.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	29.1	168.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(42.8)	(61.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	991.2	1,130.2
Total, Gross Unrealized Losses	(42.9)	(62.8)
Total, Fair Market Value	$ 1,020.3	$ 1,298.6