Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions			3 Months Ended	6 Months Ended
	May 23, 2019	Feb. 01, 2019	Jun. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued earnouts			$ 0.1	$ 0.1	$ 0.5
Performance targets, maximum potential obligation			8.9	8.9	13.3
Treasury securities			0.2	0.2	0.2
Held-to-maturity investments, carrying value			0.2	0.2	0.2
Held-to-maturity investments, fair value			0.2	0.2	0.2
Short-term investments			25.3	25.3
Purchase of short-term investments			130.0	280.0
Sale of short-term investments			105.0	255.0
Business Combinations and Asset Acquisitions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Performance targets, maximum potential obligation			0.1	0.1	$ 0.7
2018 Term Loan [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan period	4 years	4 years
Notional amount	$ 750.0	$ 1,200.0
Derivative basis spread on variable interest rate	2.00%	2.00%
Derivative fixed interest rate	4.08%	4.495%
Liability fair value of swaps			$ 51.6	$ 51.6