Corporate Investments and Funds Held For Clients (Corporate Investments And Funds Held For Clients) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	$ 1,602.2	$ 1,908.5	$ 1,500.4
Amortized Cost	21,550.4	20,699.8
Gross Unrealized Gains	802.3	877.5
Gross Unrealized Losses	(2.2)	(0.7)
Fair market value	22,350.5	21,576.6
Total corporate investments and funds held for clients - amortized cost	37,678.9	27,753.4
Total corporate investments and funds held for clients - gross unrealized gains	802.3	877.5
Total corporate investments and funds held for clients - gross unrealized losses	(2.2)	(0.7)
Total corporate investments and funds held for clients - fair market value	38,479.0	28,630.2
Corporate Investments
Debt Securities, Available-for-sale [Line Items]
Fair market value	10.6	13.6
Funds Held For Clients
Debt Securities, Available-for-sale [Line Items]
Fair market value	22,339.9	21,563.0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,266.6	9,188.7
Gross Unrealized Gains	439.7	473.4
Gross Unrealized Losses	(0.9)	0.0
Fair market value	10,705.4	9,662.1
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,741.5	3,274.6
Gross Unrealized Gains	87.3	96.0
Gross Unrealized Losses	0.0	(0.5)
Fair market value	2,828.8	3,370.1
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,609.3	3,580.6
Gross Unrealized Gains	93.8	120.8
Gross Unrealized Losses	(0.1)	0.0
Fair market value	3,703.0	3,701.4
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,403.4	1,128.2
Gross Unrealized Gains	30.4	35.6
Gross Unrealized Losses	(0.9)	0.0
Fair market value	1,432.9	1,163.8
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,084.9	1,018.7
Gross Unrealized Gains	21.8	23.1
Gross Unrealized Losses	(0.1)	0.0
Fair market value	1,106.6	1,041.8
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	805.8	814.3
Gross Unrealized Gains	54.6	53.9
Gross Unrealized Losses	0.0	0.0
Fair market value	860.4	868.2
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	713.9	676.6
Gross Unrealized Gains	34.5	33.6
Gross Unrealized Losses	(0.1)	0.0
Fair market value	748.3	710.2
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	925.0	1,018.1
Gross Unrealized Gains	40.2	41.1
Gross Unrealized Losses	(0.1)	(0.2)
Fair market value	965.1	1,059.0
Money Market Funds
Debt Securities, Available-for-sale [Line Items]
Money market securities, cash and other cash equivalents - amortized cost	16,128.5	7,053.6
Money market securities and other cash equivalents - fair market value	$ 16,128.5	$ 7,053.6

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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Amortized cost of 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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Amount of 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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