Goodwill and Identifiable Assets (Goodwill) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Gross goodwill	$ 893,776	$ 873,513	$ 874,906
Goodwill, Acquired During Period	17,493
Goodwill	893,776	873,513
Translation adjustment	2,770	(1,393)
Apex
Goodwill [Roll Forward]
Gross goodwill	662,110	644,617	644,617
Translation adjustment	0	0
Oxford
Goodwill [Roll Forward]
Gross goodwill	231,666	228,896	$ 230,289
Translation adjustment	2,770	$ (1,393)
Stratacuity [Member] | Apex
Goodwill [Roll Forward]
Goodwill, Acquired During Period	$ 17,493