Debt and Credit Agreements Credit Agreements (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 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]	3.50x
Committed credit facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 1,500.0		$ 1,000.0
Line of credit facility, available capacity increase amount		$ 250.0
Line of credit facility, expiration date		Oct. 25, 2022
Limits on letters of credit		$ 50.0
Line of credit facility, amount outstanding		0.0
Letters of credit, amount outstanding		8.4	$ 4.9
Line of credit facility, remaining borrowing capacity		1,491.6	995.1
Uncommitted credit facility
Line of Credit Facility [Line Items]
Line of credit facility, amount outstanding		84.9	85.7
Line of credit facility, current borrowing capacity		926.2	856.6
Line of credit facility, average outstanding amount		$ 223.8	$ 216.0
Line of credit facility, weighted average interest rate, over time		2.90%

[1] The interest coverage ratio is defined as EBITDA, as defined in the Credit 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 Credit Agreement, for the four quarters then ended. The leverage ratio may be changed to not more than 4.00 to 1 at our election for four consecutive fiscal quarters, beginning with the fiscal quarter in which there is an occurrence of one or more acquisitions with an aggregate purchase price of at least $200.0.