Credit Facilities (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Committed credit facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Expiration Date	Oct. 25, 2022
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,500.0
Line of Credit Facility, Available Capacity Increase Amount	250.0
Line of Credit Facility, Limit on Letters of Credit	50.0
Line of Credit Outstanding, Amount	0.0
Letters of Credit Outstanding, Amount	8.5
Line of Credit Facility, Remaining Borrowing Capacity	1,491.5
Uncommitted credit facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,150.8
Line of Credit Outstanding, Amount	178.5
Line of Credit Facility, Average Outstanding Amount	$ 111.3
Short-term Debt, Weighted Average Interest Rate, over Time	3.50%