Debt - Revolving Loan Arrangements (Details) $ in Millions	Mar. 31, 2018 USD ($)
Revolving lines of credit
Total Amount Committed by Lenders	$ 763.6
Outstanding Letters of Credit	138.8
Total Amount Available	624.8
Other lines of credit
Revolving lines of credit
Total Amount Committed by Lenders	263.6
Outstanding Letters of Credit	137.7
Total Amount Available	$ 125.9
2015 Credit Agreement | US Dollar revolving loan
Revolving lines of credit
Weighted Average Interest Rate (as a percent)	0.00%
Total Amount Committed by Lenders	$ 500.0
Outstanding Letters of Credit	1.1
Total Amount Available	$ 498.9