Corporate Investments and Funds Held for Clients - Available-for-Sale Securities That Have Been in an Unrealized Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2026	Jun. 30, 2025
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	$ (154.8)	$ (10.2)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(419.6)	(672.1)
Total, gross unrealized losses	(574.4)	(682.3)
Securities in unrealized loss position less than 12 months, fair market value	15,809.2	1,963.9
Securities in unrealized loss position greater than 12 months, fair market value	8,929.6	16,184.9
Total, fair market value	24,738.8	18,148.8
Corporate bonds
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	(83.5)	(5.4)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(258.5)	(414.8)
Total, gross unrealized losses	(342.0)	(420.2)
Securities in unrealized loss position less than 12 months, fair market value	7,318.7	690.4
Securities in unrealized loss position greater than 12 months, fair market value	6,252.1	10,218.8
Total, fair market value	13,570.8	10,909.2
U.S. Treasury securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	(51.8)	(0.8)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(5.6)	(42.9)
Total, gross unrealized losses	(57.4)	(43.7)
Securities in unrealized loss position less than 12 months, fair market value	6,176.1	732.2
Securities in unrealized loss position greater than 12 months, fair market value	308.2	1,801.7
Total, fair market value	6,484.3	2,533.9
Asset-backed securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	(8.8)	(0.2)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(6.2)	(15.3)
Total, gross unrealized losses	(15.0)	(15.5)
Securities in unrealized loss position less than 12 months, fair market value	992.6	101.4
Securities in unrealized loss position greater than 12 months, fair market value	298.1	481.4
Total, fair market value	1,290.7	582.8
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	(2.2)	(1.4)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(19.8)	(31.0)
Total, gross unrealized losses	(22.0)	(32.4)
Securities in unrealized loss position less than 12 months, fair market value	283.0	219.2
Securities in unrealized loss position greater than 12 months, fair market value	444.4	870.2
Total, fair market value	727.4	1,089.4
Canadian provincial bonds
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	(1.4)	(0.8)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(18.2)	(26.9)
Total, gross unrealized losses	(19.6)	(27.7)
Securities in unrealized loss position less than 12 months, fair market value	129.8	44.3
Securities in unrealized loss position greater than 12 months, fair market value	379.4	588.8
Total, fair market value	509.2	633.1
U.S. government agency securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	(1.3)	(0.1)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(69.2)	(79.9)
Total, gross unrealized losses	(70.5)	(80.0)
Securities in unrealized loss position less than 12 months, fair market value	75.0	8.0
Securities in unrealized loss position greater than 12 months, fair market value	659.9	1,093.8
Total, fair market value	734.9	1,101.8
Other securities
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
Securities in unrealized loss position less than 12 months, gross unrealized losses	(5.8)	(1.5)
Securities in unrealized loss position greater than 12 months, gross unrealized losses	(42.1)	(61.3)
Total, gross unrealized losses	(47.9)	(62.8)
Securities in unrealized loss position less than 12 months, fair market value	834.0	168.4
Securities in unrealized loss position greater than 12 months, fair market value	587.5	1,130.2
Total, fair market value	$ 1,421.5	$ 1,298.6