Corporate Investments and Funds Held for Clients - Corporate Investments and Funds Held for Clients (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Sep. 30, 2023
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	$ 2,104.9	$ 2,913.4	$ 1,441.4
Amortized Cost	34,324.3	32,723.3
Gross unrealized gains	293.6	30.9
Gross unrealized losses	(944.8)	(1,546.7)
Fair market value	33,673.1	31,207.5
Total corporate investments and funds held for clients - amortized cost	37,115.8	42,809.3
Total corporate investments and funds held for clients - gross unrealized gains	293.6	30.9
Total corporate investments and funds held for clients - gross unrealized losses	(944.8)	(1,546.7)
Total corporate investments and funds held for clients - fair market value	36,464.6	41,293.5
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	17,705.4	16,833.3
Gross unrealized gains	129.0	11.5
Gross unrealized losses	(578.6)	(944.8)
Fair market value	17,255.8	15,900.0
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	8,197.1	7,701.2
Gross unrealized gains	94.8	9.0
Gross unrealized losses	(81.7)	(164.5)
Fair market value	8,210.2	7,545.7
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,295.7	2,130.7
Gross unrealized gains	18.2	1.7
Gross unrealized losses	(52.2)	(86.6)
Fair market value	2,261.7	2,045.8
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,582.4	1,645.0
Gross unrealized gains	3.7	0.5
Gross unrealized losses	(98.2)	(140.6)
Fair market value	1,487.9	1,504.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,614.4	1,394.9
Gross unrealized gains	22.6	3.9
Gross unrealized losses	(25.6)	(43.0)
Fair market value	1,611.4	1,355.8
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,140.4	1,116.3
Gross unrealized gains	17.5	2.3
Gross unrealized losses	(35.6)	(56.2)
Fair market value	1,122.3	1,062.4
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	509.0	535.9
Gross unrealized gains	0.6	0.0
Gross unrealized losses	(21.7)	(35.1)
Fair market value	487.9	500.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,279.9	1,366.0
Gross unrealized gains	7.2	2.0
Gross unrealized losses	(51.2)	(75.9)
Fair market value	1,235.9	1,292.1
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	6,182.8	384.0
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Fair market value	27,490.3	30,823.5
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	2,791.5	10,086.0
Money market securities and other cash equivalents - fair market value	$ 2,791.5	$ 10,086.0