Goodwill and Identifiable Assets Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Amortization of intangible assets	$ 58,506	$ 33,444	$ 39,628
Goodwill [Roll Forward]
Gross goodwill, period start	894,095	873,513
Goodwill, Acquired During Period	528,207	17,467
Translation adjustment	(1,226)	3,115
Gross goodwill, period end	1,421,076	894,095	873,513
Apex
Goodwill [Line Items]
Amortization of intangible assets	26,185	29,361	34,359
Goodwill [Roll Forward]
Gross goodwill, period start	(662,084)	644,617
Translation adjustment	0	0
Gross goodwill, period end	(662,084)	(662,084)	644,617
Oxford
Goodwill [Line Items]
Amortization of intangible assets	4,179	4,083	5,269
Goodwill [Roll Forward]
Gross goodwill, period start	(232,011)	228,896
Translation adjustment	(1,226)	3,115
Gross goodwill, period end	(230,785)	(232,011)	$ 228,896
ECS
Goodwill [Line Items]
Amortization of intangible assets	28,142
Goodwill [Roll Forward]
Goodwill, Acquired During Period	528,207
Gross goodwill, period end	$ 528,207
Stratacuity | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 17,467