Debt - Revolving Loan Arrangements (Details) $ in Millions	Sep. 30, 2018 USD ($)
Revolving lines of credit
Total Amount Committed by Lenders	$ 750.8
Outstanding Borrowings	20.0
Outstanding Letters of Credit	120.7
Total Amount Available	610.1
Other lines of credit
Revolving lines of credit
Total Amount Committed by Lenders	250.8
Outstanding Letters of Credit	119.6
Total Amount Available	$ 131.2
2015 Credit Agreement | US Dollar revolving loan
Revolving lines of credit
Weighted Average Interest Rate (as a percent)	3.20%
Total Amount Committed by Lenders	$ 500.0
Outstanding Borrowings	20.0
Outstanding Letters of Credit	1.1
Total Amount Available	$ 478.9