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Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Apr. 02, 2018	Dec. 31, 2016
Goodwill [Roll Forward]
Gross goodwill	$ 1,420,636	$ 894,095		$ 873,513
Goodwill, Acquired During Period	527,388	17,467
Goodwill	1,420,636	894,095	$ 527,388
Translation adjustment	(847)	3,115
Apex
Goodwill [Roll Forward]
Gross goodwill	662,084	662,084		644,617
Translation adjustment		0
Oxford
Goodwill [Roll Forward]
Gross goodwill	231,164	232,011		$ 228,896
Translation adjustment	(847)	3,115
ECS
Goodwill [Roll Forward]
Gross goodwill	527,388
Stratacuity [Member] \| Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 17,467
ECS \| ECS
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 527,388