Corporate Investments and Funds Held for Clients - Corporate Investments and Funds Held for Clients (Details) - USD ($) $ in Millions	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	$ 2,680.6	$ 2,913.4	$ 3,291.7
Amortized Cost	34,544.6	32,723.3
Gross Unrealized  Gains	168.2	30.9
Gross Unrealized Losses	(856.2)	(1,546.7)
Fair market value	33,856.6	31,207.5
Total corporate investments and funds held for clients - amortized cost	42,743.8	42,809.3
Total corporate investments and funds held for clients - gross unrealized gains	168.2	30.9
Total corporate investments and funds held for clients - gross unrealized losses	(856.2)	(1,546.7)
Total corporate investments and funds held for clients - fair market value	42,055.8	41,293.5
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,602.6	16,833.3
Gross Unrealized  Gains	56.3	11.5
Gross Unrealized Losses	(536.7)	(944.8)
Fair market value	17,122.2	15,900.0
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,494.6	7,701.2
Gross Unrealized  Gains	56.1	9.0
Gross Unrealized Losses	(62.5)	(164.5)
Fair market value	8,488.2	7,545.7
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,167.1	2,130.7
Gross Unrealized  Gains	21.1	1.7
Gross Unrealized Losses	(37.0)	(86.6)
Fair market value	2,151.2	2,045.8
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,588.4	1,645.0
Gross Unrealized  Gains	2.0	0.5
Gross Unrealized Losses	(91.8)	(140.6)
Fair market value	1,498.6	1,504.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,788.1	1,394.9
Gross Unrealized  Gains	15.2	3.9
Gross Unrealized Losses	(21.1)	(43.0)
Fair market value	1,782.2	1,355.8
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,131.8	1,116.3
Gross Unrealized  Gains	14.6	2.3
Gross Unrealized Losses	(31.6)	(56.2)
Fair market value	1,114.8	1,062.4
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	467.2	535.9
Gross Unrealized  Gains	0.4	0.0
Gross Unrealized Losses	(22.0)	(35.1)
Fair market value	445.6	500.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,304.8	1,366.0
Gross Unrealized  Gains	2.5	2.0
Gross Unrealized Losses	(53.5)	(75.9)
Fair market value	1,253.8	1,292.1
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Fair market value	33,856.6	30,823.5
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	0.0	384.0
Money market securities, cash and other cash equivalents
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	8,199.2	10,086.0
Money market securities and other cash equivalents - fair market value	$ 8,199.2	$ 10,086.0