Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Roll Forward]
Gross goodwill	$ 875,015	$ 874,906	$ 512,060
Goodwill	875,015	874,906
Translation adjustment	109	(1,632)
Apex
Goodwill [Roll Forward]
Gross goodwill	644,617	644,617	287,951
Translation adjustment	0	(1,363)
Oxford
Goodwill [Roll Forward]
Gross goodwill	230,398	230,289	$ 224,109
Translation adjustment	$ 109	(269)
Creative Circle [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period		358,029
Creative Circle [Member] | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period		358,029
Creative Circle [Member] | Oxford
Goodwill [Roll Forward]
Goodwill, Acquired During Period		0
LabResource [Member]
Goodwill [Roll Forward]
Goodwill, Acquired During Period		6,449
LabResource [Member] | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period		0
LabResource [Member] | Oxford
Goodwill [Roll Forward]
Goodwill, Acquired During Period		$ 6,449