Corporate Investments and Funds Held For Clients (Available-For-Sale Securities That Have Been In An Unrealized Loss Position) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	$ (81.9)	$ (73.5)
Fair market value of securities in unrealized loss position less than 12 months	10,372.8	8,915.3
Unrealized losses greater than 12 months	(69.3)	(0.3)
Fair market value of securities in unrealized loss positions greater than 12 months	4,004.3	29.8
Total gross unrealized losses	(151.2)	(73.8)
Total fair market value	14,377.1	8,945.1
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(18.3)	(22.0)
Fair market value of securities in unrealized loss position less than 12 months	3,437.6	2,619.9
Unrealized losses greater than 12 months	(21.4)	0.0
Fair market value of securities in unrealized loss positions greater than 12 months	1,186.2	7.4
Total gross unrealized losses	(39.7)	(22.0)
Total fair market value	4,623.8	2,627.3
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(10.8)	(8.5)
Fair market value of securities in unrealized loss position less than 12 months	2,463.5	1,916.1
Unrealized losses greater than 12 months	(11.2)	(0.1)
Fair market value of securities in unrealized loss positions greater than 12 months	1,065.1	11.3
Total gross unrealized losses	(22.0)	(8.6)
Total fair market value	3,528.6	1,927.4
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(9.6)	(13.4)
Fair market value of securities in unrealized loss position less than 12 months	1,390.0	1,935.3
Unrealized losses greater than 12 months	(13.2)	0.0
Fair market value of securities in unrealized loss positions greater than 12 months	656.1	0.0
Total gross unrealized losses	(22.8)	(13.4)
Total fair market value	2,046.1	1,935.3
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(17.1)	(14.3)
Fair market value of securities in unrealized loss position less than 12 months	1,412.9	1,317.8
Unrealized losses greater than 12 months	(21.5)	0.0
Fair market value of securities in unrealized loss positions greater than 12 months	982.1	1.0
Total gross unrealized losses	(38.6)	(14.3)
Total fair market value	2,395.0	1,318.8
Canadian government obligations and Canadian government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(20.1)	(11.4)
Fair market value of securities in unrealized loss position less than 12 months	895.3	699.6
Unrealized losses greater than 12 months	0.0	0.0
Fair market value of securities in unrealized loss positions greater than 12 months	0.0	0.0
Total gross unrealized losses	(20.1)	(11.4)
Total fair market value	895.3	699.6
Canadian provincial bonds
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(2.7)	(1.4)
Fair market value of securities in unrealized loss position less than 12 months	315.1	179.8
Unrealized losses greater than 12 months	(0.6)	0.0
Fair market value of securities in unrealized loss positions greater than 12 months	55.4	0.0
Total gross unrealized losses	(3.3)	(1.4)
Total fair market value	370.5	179.8
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(1.4)	(1.2)
Fair market value of securities in unrealized loss position less than 12 months	189.2	98.8
Unrealized losses greater than 12 months	(0.6)	(0.1)
Fair market value of securities in unrealized loss positions greater than 12 months	17.8	1.2
Total gross unrealized losses	(2.0)	(1.3)
Total fair market value	207.0	100.0
Other securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses less than 12 months	(1.9)	(1.3)
Fair market value of securities in unrealized loss position less than 12 months	269.2	148.0
Unrealized losses greater than 12 months	(0.8)	(0.1)
Fair market value of securities in unrealized loss positions greater than 12 months	41.6	8.9
Total gross unrealized losses	(2.7)	(1.4)
Total fair market value	$ 310.8	$ 156.9