Pro Forma Financial Information (re: 2017 and 2016 acquisitions) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition, Pro Forma Information [Abstract]
Total revenues	$ 104,681	$ 92,800	$ 87,387	$ 79,103	$ 80,046	$ 74,608	$ 72,574	$ 71,066	$ 70,204	$ 66,813	$ 64,712	$ 63,753	$ 363,971	$ 298,294	$ 265,482
Net Income, As Reported	$ 26,573	$ 24,184	$ 23,434	$ 26,427	$ 24,629	$ 24,067	$ 22,992	$ 22,159	$ 21,929	$ 20,232	$ 19,036	$ 18,336	$ 100,618	$ 93,847	$ 79,533
Basic EPS, As Reported (in dollars per share)	$ 0.84	$ 0.77	$ 0.74	$ 0.83	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 3.19	$ 2.88	$ 2.29
Diluted EPS, As Reported (in dollars per share)	$ 0.84	$ 0.76	$ 0.74	$ 0.83	$ 0.76	$ 0.74	$ 0.70	$ 0.67	$ 0.65	$ 0.59	$ 0.54	$ 0.51	$ 3.17	$ 2.86	$ 2.28
Increase (decrease) in reported revenue													$ 65,700
Increase (decrease) in reported revenue, percentage													22.00%
Effect of Exchange Rate on Revenue													$ 2,100	$ (3,300)
Parent, Curepet, Healthcare Magic, Vertex, Oakstone, I3, Qatarlyst
Business Acquisition, Pro Forma Information [Abstract]
Revenue, Pro forma													440,512	434,152
Net Income, Pro Forma													$ 104,851	$ 96,147
Basic EPS, Pro Forma (in dollars per share)													$ 3.32	$ 2.95
Diluted EPS, Pro Forma (in dollars per share)													$ 3.31	$ 2.93
Increase (decrease) in unaudited pro forma revenue													$ 6,400
Increase (decrease) in unaudited pro forma revenue, percentage													1.50%
Exchange rate effect on reported revenues													$ 2,100