Corporate Investments and Funds Held for Clients - Schedule of Corporate Investments and Funds Held for Clients (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025	Sep. 30, 2024
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	$ 2,478.0	$ 3,347.8	$ 2,104.9
Amortized Cost	35,863.8	34,203.4
Gross Unrealized  Gains	333.3	256.6
Gross Unrealized Losses	(544.8)	(682.3)
Fair market value	35,652.3	33,777.7
Total corporate investments and funds held for clients - amortized cost	39,855.5	39,258.0
Total corporate investments and funds held for clients - gross unrealized gains	333.3	256.6
Total corporate investments and funds held for clients - gross unrealized losses	(544.8)	(682.3)
Total corporate investments and funds held for clients - fair market value	39,644.0	38,832.3
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,012.4	17,515.2
Gross Unrealized  Gains	148.2	108.7
Gross Unrealized Losses	(333.7)	(420.2)
Fair market value	16,826.9	17,203.7
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,874.5	8,416.8
Gross Unrealized  Gains	103.6	85.6
Gross Unrealized Losses	(32.3)	(43.7)
Fair market value	10,945.8	8,458.7
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,060.9	1,837.8
Gross Unrealized  Gains	28.5	22.6
Gross Unrealized Losses	(11.1)	(15.5)
Fair market value	2,078.3	1,844.9
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,747.7	1,972.0
Gross Unrealized  Gains	19.2	16.4
Gross Unrealized Losses	(25.0)	(32.4)
Fair market value	1,741.9	1,956.0
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,123.1	1,155.8
Gross Unrealized  Gains	18.0	13.8
Gross Unrealized Losses	(22.6)	(27.7)
Fair market value	1,118.5	1,141.9
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,007.4	1,328.5
Gross Unrealized  Gains	4.4	2.3
Gross Unrealized Losses	(69.0)	(80.0)
Fair market value	942.8	1,250.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,037.8	1,977.3
Gross Unrealized  Gains	11.4	7.2
Gross Unrealized Losses	(51.1)	(62.8)
Fair market value	1,998.1	1,921.7
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	5,466.1	4,498.8
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Fair market value	30,186.2	29,278.9
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	3,991.7	5,054.6
Money market securities and other cash equivalents - fair market value	$ 3,991.7	$ 5,054.6