Corporate Investments and Funds Held For Clients (Available-For-Sale Securities That Have Been In An Unrealized Loss Position) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (111.0)	$ (77.6)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	9,497.6	6,331.3
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(7.2)	(0.1)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	367.6	1.9
Total, Gross Unrealized Losses	(118.2)	(77.7)
Total, Fair Market Value	9,865.2	6,333.2
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(60.7)	(38.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	5,522.8	3,539.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(1.8)	0.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	50.5	0.0
Total, Gross Unrealized Losses	(62.5)	(38.5)
Total, Fair Market Value	5,573.3	3,539.6
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(14.6)	(9.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	1,515.5	580.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	0.0	0.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	0.0	0.0
Total, Gross Unrealized Losses	(14.6)	(9.3)
Total, Fair Market Value	1,515.5	580.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(2.1)	(0.9)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	280.6	160.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	0.0	0.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	0.0	0.0
Total, Gross Unrealized Losses	(2.1)	(0.9)
Total, Fair Market Value	280.6	160.9
Canadian government obligations and Canadian government agency obligations
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(11.8)	(9.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	944.4	721.3
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	0.0	0.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	0.0	0.0
Total, Gross Unrealized Losses	(11.8)	(9.0)
Total, Fair Market Value	944.4	721.3
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(7.1)	(9.4)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	496.3	749.7
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(5.3)	0.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	315.2	0.0
Total, Gross Unrealized Losses	(12.4)	(9.4)
Total, Fair Market Value	811.5	749.7
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(6.5)	(5.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	299.5	253.7
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	0.0	0.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	0.0	0.0
Total, Gross Unrealized Losses	(6.5)	(5.3)
Total, Fair Market Value	299.5	253.7
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.6)	0.0
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	86.6	16.7
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	0.0	0.0
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	0.0	0.0
Total, Gross Unrealized Losses	(0.6)	0.0
Total, Fair Market Value	86.6	16.7
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(7.6)	(5.2)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	351.9	308.5
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(0.1)	(0.1)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1.9	1.9
Total, Gross Unrealized Losses	(7.7)	(5.3)
Total, Fair Market Value	$ 353.8	$ 310.4

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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