Corporate Investments and Funds Held For Clients (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018		Jun. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Earliest Corporate bond debt maturity date	October 2018
Latest corporate debt maturity date	September 2028
Available-for-sale securities - Fair Market Value	$ 22,947.6	[1]	$ 22,776.2	[2]
Earliest U.S. Treasury maturity date	October 2018
Latest U.S Treasury maturity date	August 2026
Client Fund obligation repayment period	1 year
Client funds obligations	$ 25,798.9		27,493.5
Client funds held in grantor trust	$ 22,994.7		24,242.9
Client funds investments with original maturities	ninety days or less
Percentage of the available-for-sale securities were rated AAA Or AA	79.00%
Federal Home Loan Banks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	$ 1,785.7
Federal Farm Credit Banks
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	678.5
U.S. government agency securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	125.4
Fixed Rate Credit Card
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	2,018.3
Asset-Backed Auto Loan Receivables
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	1,799.5
Asset-Backed Equipment Lease Receivable
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	467.6
Rate Reduction
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	189.1
Supranational Bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	195.2
Commercial Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	307.8
Sovereign Bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	109.8
Level 1
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	0.0		0.0
Level 3
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities - Fair Market Value	$ 0.0		$ 0.0

[1] Included within available-for-sale securities are corporate investments with fair values of $462.9 million and funds held for clients with fair values of $22,484.7 million. All available-for-sale securities were included in Level 2 of the fair value hierarchy.

[2] Included within available-for-sale securities are corporate investments with fair values of $10.5 million and funds held for clients with fair values of $22,765.7 million. All available-for-sale securities were included in Level 2 of the fair value hierarchy.