Description of Business and Summary of Significant Accounting Policies (Assets and Liabilities Measured at Fair Value on a Recurring Basis using Significant Unobservable Inputs) (Details) - USD ($)		9 Months Ended		12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation:
Beginning balance		$ 37,096,000	$ 8,510,000	$ 8,510,000
(Gains) or losses included in earnings	[1]	(645,000)		(164,000)
Goodwill, Translation and Purchase Accounting Adjustments	[1]	(13,718,000)	$ (3,432,000)	(4,007,000)
Foreign currency translation adjustments	[2]	(2,843,000)		522,000
Business acquisitions		13,226,000		34,156,000
Settlements		(3,831,000)		(1,921,000)
Ending balance		29,285,000		37,096,000
The amount of total (gains) losses for the period included in earnings or changes to net assets, attributable to changes in unrealized gains relating to assets or liabilities still held at period-end.		$ (645,000)		$ 0

[1] Recorded as an adjustment to reported general and administrative expenses

[2] Recorded as a component of other comprehensive income within stockholders' equity