Fair Value Measurements (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued earnouts	$ 2.5	$ 4.1
Performance targets, maximum potential obligation	11.1
Treasury securities	0.2	0.2
Held-to-maturity investments, carrying value	0.5	0.7
Held-to-maturity investments, fair value	0.5	0.7
Business Combinations and Asset Acquisitions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Performance targets, maximum potential obligation	$ 3.1	$ 5.8
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate under Revolving Credit Facility	1.375%
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Interest rate under Revolving Credit Facility	2.00%