Debt and Credit Agreements Credit Agreements (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Margin for base rate advances			0.10%
Margin for Eurocurrency rate advances			1.10%
Annual rate for fronting fees accrued on letters of credit			0.25%
Annual facility fee			0.15%
Interest coverage ratio minimum	[1]		5.00x
Leverage ratio maximum	[2]		4.00x
Quarters 1-3 Maximum Leverage Ratio Covenant		4.00 to 1.00
Quarters 4-7 Maximum Leverage Ratio Covenant		3.75 to 1.00
Quarters 8-Thereafter Maximum Leverage Ratio Covenant		3.50 to 1.00
Committed credit facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity			$ 1,500.0
Line of credit facility, available capacity increase amount			250.0
Limits on letters of credit			50.0
Line of credit facility, amount outstanding			0.0
Letters of credit, amount outstanding			8.5	$ 8.4
Line of credit facility, remaining borrowing capacity			1,491.5	1,491.6
Uncommitted credit facility
Line of Credit Facility [Line Items]
Line of credit facility, amount outstanding			73.7	84.9
Line of credit facility, current borrowing capacity			1,173.1	926.2
Line of credit facility, average outstanding amount			$ 96.4	$ 223.8
Line of credit facility, weighted average interest rate, over time			4.50%	2.90%

[1] The interest coverage ratio is defined as EBITDA, as defined in the Credit Agreement and Term Loan Agreement, to net interest expense for the four quarters then ended.

[2] The leverage ratio is defined as debt as of the last day of such fiscal quarter to EBITDA, as defined in the Amended Credit Agreement and Term Loan Agreement, for the four quarters then ended.