Corporate Investments and Funds Held for Clients (Corporate Investments and Funds Held for Clients) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Jun. 30, 2023	Sep. 30, 2022
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	$ 1,441.4	$ 2,083.5	$ 1,207.7
Amortized Cost	31,956.0	31,971.8
Gross Unrealized  Gains	0.2	7.5
Gross Unrealized Losses	(2,355.8)	(2,214.4)
Fair market value	29,600.4	29,764.9
Total corporate investments and funds held for clients - amortized cost	38,917.9	40,743.3
Total corporate investments and funds held for clients - gross unrealized gains	0.2	7.5
Total corporate investments and funds held for clients - gross unrealized losses	(2,355.8)	(2,214.4)
Total corporate investments and funds held for clients - fair market value	36,562.3	38,536.4
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,836.9	15,870.7
Gross Unrealized  Gains	0.1	4.7
Gross Unrealized Losses	(1,407.3)	(1,308.3)
Fair market value	14,429.7	14,567.1
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,213.5	8,054.7
Gross Unrealized  Gains	0.0	0.7
Gross Unrealized Losses	(283.8)	(290.4)
Fair market value	7,929.7	7,765.0
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,049.8	2,070.4
Gross Unrealized  Gains	0.0	0.0
Gross Unrealized Losses	(149.2)	(145.0)
Fair market value	1,900.6	1,925.4
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,669.6	1,670.0
Gross Unrealized  Gains	0.0	0.2
Gross Unrealized Losses	(197.9)	(179.8)
Fair market value	1,471.7	1,490.4
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,136.6	1,234.7
Gross Unrealized  Gains	0.0	0.0
Gross Unrealized Losses	(68.1)	(69.7)
Fair market value	1,068.5	1,165.0
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,013.1	1,000.5
Gross Unrealized  Gains	0.0	0.2
Gross Unrealized Losses	(88.0)	(78.1)
Fair market value	925.1	922.6
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	661.7	679.2
Gross Unrealized  Gains	0.0	0.0
Gross Unrealized Losses	(51.9)	(46.7)
Fair market value	609.8	632.5
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,374.8	1,391.6
Gross Unrealized  Gains	0.1	1.7
Gross Unrealized Losses	(109.6)	(96.4)
Fair market value	1,265.3	1,296.9
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	358.0	119.3
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Fair market value	29,242.4	29,645.6
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	6,961.9	8,771.5
Money market securities and other cash equivalents - fair market value	$ 6,961.9	$ 8,771.5