Corporate Investments and Funds Held for Clients - Corporate Investments and Funds Held for Clients (Details) - USD ($) $ in Millions	Mar. 31, 2024	Jun. 30, 2023	Mar. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	$ 3,291.7	$ 2,083.5	$ 1,828.4
Amortized Cost	32,679.3	31,971.8
Gross Unrealized  Gains	31.3	7.5
Gross Unrealized Losses	(1,614.2)	(2,214.4)
Fair market value	31,096.4	29,764.9
Total corporate investments and funds held for clients - amortized cost	52,804.1	40,743.3
Total corporate investments and funds held for clients - gross unrealized gains	31.3	7.5
Total corporate investments and funds held for clients - gross unrealized losses	(1,614.2)	(2,214.4)
Total corporate investments and funds held for clients - fair market value	51,221.2	38,536.4
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	16,425.9	15,870.7
Gross Unrealized  Gains	12.7	4.7
Gross Unrealized Losses	(975.2)	(1,308.3)
Fair market value	15,463.4	14,567.1
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,214.9	8,054.7
Gross Unrealized  Gains	7.6	0.7
Gross Unrealized Losses	(178.4)	(290.4)
Fair market value	8,044.1	7,765.0
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,108.2	2,070.4
Gross Unrealized  Gains	0.5	0.0
Gross Unrealized Losses	(99.1)	(145.0)
Fair market value	2,009.6	1,925.4
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,637.5	1,670.0
Gross Unrealized  Gains	0.1	0.2
Gross Unrealized Losses	(145.4)	(179.8)
Fair market value	1,492.2	1,490.4
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,278.1	1,234.7
Gross Unrealized  Gains	4.1	0.0
Gross Unrealized Losses	(45.8)	(69.7)
Fair market value	1,236.4	1,165.0
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,076.8	1,000.5
Gross Unrealized  Gains	2.1	0.2
Gross Unrealized Losses	(59.4)	(78.1)
Fair market value	1,019.5	922.6
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	574.0	679.2
Gross Unrealized  Gains	0.0	0.0
Gross Unrealized Losses	(36.0)	(46.7)
Fair market value	538.0	632.5
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,363.9	1,391.6
Gross Unrealized  Gains	4.2	1.7
Gross Unrealized Losses	(74.9)	(96.4)
Fair market value	1,293.2	1,296.9
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Fair market value		119.3
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Fair market value	31,096.4	29,645.6
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	20,124.8	8,771.5
Money market securities and other cash equivalents - fair market value	$ 20,124.8	$ 8,771.5