Debt (2016 Senior Revolving Credit Agreement) (Details) - 2016 Senior Revolving Credit Facility - Line of Credit - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Nov. 22, 2016
Line of Credit Facility [Line Items]
Line of credit base interest rate	0.20%
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,000.0
Line of Credit Facility, Interest Rate During Period	1.20%
Line of Credit Facility, Commitment Fee Percentage	0.175%
Line of Credit Facility, Fair Value of Amount Outstanding	$ 0.0
Trailing Four Quarters
Line of Credit Facility [Line Items]
Debt Instrument, Covenant Description	3.75 to 1.00