Corporate Investments and Funds Held For Clients - Available-For-Sale Securities That Have Been In An Unrealized Loss Position (Details) - USD ($) $ in Millions	Jun. 30, 2023	Jun. 30, 2022
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	$ (179.8)	$ (1,305.8)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	8,235.0	20,798.8
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(2,034.6)	(425.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	20,814.5	4,005.8
Total, Gross Unrealized Losses	(2,214.4)	(1,731.1)
Total, Fair Market Value	29,049.5	24,804.6
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(62.0)	(824.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	2,255.9	11,525.4
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(1,246.3)	(259.0)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	12,050.5	2,356.7
Total, Gross Unrealized Losses	(1,308.3)	(1,083.0)
Total, Fair Market Value	14,306.4	13,882.1
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(85.5)	(126.4)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	4,629.4	2,919.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(204.9)	(44.7)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	2,876.3	464.6
Total, Gross Unrealized Losses	(290.4)	(171.1)
Total, Fair Market Value	7,505.7	3,384.2
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	(5.8)	(110.0)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	333.9	1,782.6
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(139.2)	(13.5)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,588.0	113.3
Total, Gross Unrealized Losses	(145.0)	(123.5)
Total, Fair Market Value	1,921.9	1,895.9
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(0.6)	(75.3)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	28.2	859.3
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(179.2)	(62.9)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	1,432.2	695.6
Total, Gross Unrealized Losses	(179.8)	(138.2)
Total, Fair Market Value	1,460.4	1,554.9
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(2.0)	(52.6)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	159.7	1,444.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(67.7)	(6.2)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	975.6	59.9
Total, Gross Unrealized Losses	(69.7)	(58.8)
Total, Fair Market Value	1,135.3	1,504.8
Canadian provincial bonds
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(2.7)	(45.4)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	127.0	726.9
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(75.4)	(17.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	757.3	133.2
Total, Gross Unrealized Losses	(78.1)	(62.7)
Total, Fair Market Value	884.3	860.1
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(6.7)	(29.5)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	126.9	802.8
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(40.0)	(0.4)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	505.6	4.3
Total, Gross Unrealized Losses	(46.7)	(29.9)
Total, Fair Market Value	632.5	807.1
Other securities
Debt Securities, Available-for-sale [Line Items]
Securities in Unrealized Loss Position Less Than 12 Months, Gross Unrealized Losses	(14.5)	(42.6)
Securities in Unrealized Loss Position Less Than 12 Months, Fair Market Value	574.0	737.3
Securities in Unrealized Loss Position Greater Than 12 Months, Gross Unrealized Losses	(81.9)	(21.3)
Securities in Unrealized Loss Position Greater Than 12 Months, Fair Market Value	629.0	178.2
Total, Gross Unrealized Losses	(96.4)	(63.9)
Total, Fair Market Value	$ 1,203.0	$ 915.5