Quarterly Results of Operations (Unaudited) (Results of Operations) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 222,942	$ 226,863	$ 214,675	$ 214,611	$ 209,008	$ 206,783	$ 201,424	$ 203,828	$ 879,091	$ 821,043	$ 656,314
Gross margin	78,375	79,130	76,672	76,606	75,491	82,188	80,546	80,350	$ 310,783	$ 318,575	$ 249,485
Net income	$ 24,452	$ 19,734	$ 17,329	$ 26,136	$ 6,999	$ 25,532	$ 20,896	$ 27,032
Net income per share:
Basic (in dollars per share)	$ 1.33	$ 1.07	$ 0.94	$ 1.43	$ 0.38	$ 1.40	$ 1.15	$ 1.50
Diluted (in dollars per share)	$ 1.31	$ 1.06	$ 0.93	$ 1.40	$ 0.37	$ 1.37	$ 1.13	$ 1.47