Description of Business and Summary of Significant Accounting Policies (Goodwill) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2017
Business Combination Segment Allocation [Line Items]
Goodwill	$ 666,863	$ 441,404	$ 666,863
Goodwill [Roll Forward]
Beginning Balance	666,863	441,404
Goodwill, Acquired During Period	192,271	233,095
Goodwill, Purchase Accounting Adjustments	(13,071)	(12,158)
Foreign currency translation adjustments	(20,537)	4,522
Ending Balance	825,526	666,863
RCS [Member]
Business Combination Segment Allocation [Line Items]
Reporting Unit, Amount of Fair Value in Excess of Carrying Amount			$ 42,400
Reporting Unit, Percentage of Fair Value in Excess of Carrying Amount			37.00%
Reporting Unit, Assessed Fair Value			$ 158,000
Reporting Unit, Carrying Value			115,600
Goodwill	78,200	$ 78,200	$ 78,200
Fair Value Input, Discount Rate		16.00%
Goodwill [Roll Forward]
Beginning Balance	$ 78,200
Ending Balance		$ 78,200
Minimum [Member] | RCS [Member]
Business Combination Segment Allocation [Line Items]
Fair Value Input, Long-term Revenue Growth Rate		7.50%
Maximum [Member] | RCS [Member]
Business Combination Segment Allocation [Line Items]
Fair Value Input, Long-term Revenue Growth Rate		12.50%