Corporate Investments and Funds Held for Clients - Schedule of Corporate Investments and Funds Held for Clients (Details) - USD ($) $ in Millions	Dec. 31, 2025	Jun. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - Amortized Cost	$ 2,424.7	$ 3,347.8	$ 2,216.4
Available-for-sale, Amortized Cost	36,422.5	34,203.4
Available-for-sale, Gross Unrealized Gains	349.4	256.6
Available-for-sale, Gross Unrealized Losses	(453.1)	(682.3)
Available-for-sale, Fair Market Value	36,318.8	33,777.7
Total corporate investments and funds held for clients - Amortized Cost	70,084.2	39,258.0
Total corporate investments and funds held for clients - Gross Unrealized Gains	349.4	256.6
Total corporate investments and funds held for clients - Gross Unrealized Losses	(453.1)	(682.3)
Total corporate investments and funds held for clients - Fair Market Value	69,980.5	38,832.3
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Amortized Cost	17,695.5	17,515.2
Available-for-sale, Gross Unrealized Gains	156.4	108.7
Available-for-sale, Gross Unrealized Losses	(276.2)	(420.2)
Available-for-sale, Fair Market Value	17,575.7	17,203.7
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Amortized Cost	10,893.0	8,416.8
Available-for-sale, Gross Unrealized Gains	114.2	85.6
Available-for-sale, Gross Unrealized Losses	(17.7)	(43.7)
Available-for-sale, Fair Market Value	10,989.5	8,458.7
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Amortized Cost	2,074.6	1,837.8
Available-for-sale, Gross Unrealized Gains	30.7	22.6
Available-for-sale, Gross Unrealized Losses	(8.4)	(15.5)
Available-for-sale, Fair Market Value	2,096.9	1,844.9
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Amortized Cost	1,719.3	1,972.0
Available-for-sale, Gross Unrealized Gains	13.9	16.4
Available-for-sale, Gross Unrealized Losses	(23.9)	(32.4)
Available-for-sale, Fair Market Value	1,709.3	1,956.0
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Amortized Cost	1,131.7	1,155.8
Available-for-sale, Gross Unrealized Gains	15.3	13.8
Available-for-sale, Gross Unrealized Losses	(20.4)	(27.7)
Available-for-sale, Fair Market Value	1,126.6	1,141.9
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Amortized Cost	905.4	1,328.5
Available-for-sale, Gross Unrealized Gains	4.6	2.3
Available-for-sale, Gross Unrealized Losses	(63.6)	(80.0)
Available-for-sale, Fair Market Value	846.4	1,250.8
Other securities
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Amortized Cost	2,003.0	1,977.3
Available-for-sale, Gross Unrealized Gains	14.3	7.2
Available-for-sale, Gross Unrealized Losses	(42.9)	(62.8)
Available-for-sale, Fair Market Value	1,974.4	1,921.7
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Fair Market Value	45.7	4,498.8
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Available-for-sale, Fair Market Value	36,273.1	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	33,661.7	5,054.6
Money market securities and other cash equivalents - Fair Market Value	$ 33,661.7	$ 5,054.6

Total corporate investments and funds held for clients - amortized cost

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Total corporate investments and funds held for clients - fair value

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Total corporate investments and funds held for clients - gross unrealized gains

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Total corporate investments and funds held for clients - gross unrealized losses

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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