Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued earnouts	$ 500	$ 2,500
Performance targets, maximum potential obligation	13,300	11,100
Treasury securities	200	200
Held-to-maturity investments, carrying value	200	500
Held-to-maturity investments, fair value	200	500
Purchase of investments	(156,983)	(397)	$ (100)
Proceeds from sale of investments	150,890	231	$ 712
Business Combinations and Asset Acquisitions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Performance targets, maximum potential obligation	$ 700	$ 3,100
Revolving Credit Facility [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis spread on variable interest rate	1.125%
Revolving Credit Facility [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Basis spread on variable interest rate	1.75%