Corporate Investments and Funds Held for Clients - Corporate Investments and Funds Held for Clients (Details) - USD ($) $ in Millions	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	$ 1,641.3	$ 2,083.5	$ 1,345.0
Amortized Cost	31,788.6	31,971.8
Gross Unrealized  Gains	67.7	7.5
Gross Unrealized Losses	(1,560.8)	(2,214.4)
Fair market value	30,295.5	29,764.9
Total corporate investments and funds held for clients - amortized cost	45,667.9	40,743.3
Total corporate investments and funds held for clients - gross unrealized gains	67.7	7.5
Total corporate investments and funds held for clients - gross unrealized losses	(1,560.8)	(2,214.4)
Total corporate investments and funds held for clients - fair market value	44,174.8	38,536.4
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,872.1	15,870.7
Gross Unrealized  Gains	28.9	4.7
Gross Unrealized Losses	(934.8)	(1,308.3)
Fair market value	14,966.2	14,567.1
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,030.1	8,054.7
Gross Unrealized  Gains	17.8	0.7
Gross Unrealized Losses	(174.7)	(290.4)
Fair market value	7,873.2	7,765.0
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,109.2	2,070.4
Gross Unrealized  Gains	2.1	0.0
Gross Unrealized Losses	(101.6)	(145.0)
Fair market value	2,009.7	1,925.4
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,661.1	1,670.0
Gross Unrealized  Gains	0.3	0.2
Gross Unrealized Losses	(144.6)	(179.8)
Fair market value	1,516.8	1,490.4
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,152.4	1,234.7
Gross Unrealized  Gains	6.2	0.0
Gross Unrealized Losses	(46.5)	(69.7)
Fair market value	1,112.1	1,165.0
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	992.5	1,000.5
Gross Unrealized  Gains	4.7	0.2
Gross Unrealized Losses	(55.8)	(78.1)
Fair market value	941.4	922.6
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	603.5	679.2
Gross Unrealized  Gains	0.0	0.0
Gross Unrealized Losses	(34.7)	(46.7)
Fair market value	568.8	632.5
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,367.7	1,391.6
Gross Unrealized  Gains	7.7	1.7
Gross Unrealized Losses	(68.1)	(96.4)
Fair market value	1,307.3	1,296.9
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	24.4	119.3
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Fair market value	30,271.1	29,645.6
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	13,879.3	8,771.5
Money market securities and other cash equivalents - fair market value	$ 13,879.3	$ 8,771.5