Pro Forma Financial Information (re: 2016 and 2015 acquisitions) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Business Acquisition, Pro Forma Information [Abstract]
Total revenues	$ 80,046	$ 74,608	$ 72,574	$ 71,066	$ 70,204	$ 66,813	$ 64,712	$ 63,753	$ 60,633	$ 50,808	$ 51,476	$ 51,404	$ 298,294	$ 265,482	$ 214,321
Net Income, As Reported	$ 24,629	$ 24,067	$ 22,992	$ 22,159	$ 21,929	$ 20,232	$ 19,036	$ 18,336	$ 16,547	$ 18,015	$ 13,579	$ 15,417	$ 93,847	$ 79,533	$ 63,558
Basic EPS, As Reported (in dollars per share)	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 0.45	$ 0.47	$ 0.35	$ 0.40	$ 2.88	$ 2.29	$ 1.68
Diluted EPS, As Reported (in dollars per share)	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 0.45	$ 0.47	$ 0.35	$ 0.40	$ 2.86	$ 2.28	$ 1.67
Increase (decrease) in reported revenue													$ 32,800
Increase (decrease) in reported revenue, percentage													12.40%
Effect of Exchange Rate on Revenue													$ (3,300)	$ (10,700)	$ (3,200)
Parent, Curepet, Healthcare Magic, Vertex, Oakstone, I3, Qatarlyst
Business Acquisition, Pro Forma Information [Abstract]
Revenue, Pro forma													315,820	295,290
Net Income, Pro Forma													$ 95,326	$ 84,624
Basic EPS, Pro Forma (in dollars per share)													$ 2.92	$ 2.44
Diluted EPS, Pro Forma (in dollars per share)													$ 2.90	$ 2.42
Increase (decrease) in unaudited pro forma revenue													$ 20,500
Increase (decrease) in unaudited pro forma revenue, percentage													7.00%
Exchange rate effect on reported revenues													$ (3,300)