Quarterly Results Of Operations (Unaudited) (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	$ 536,616	$ 604,244	$ 684,455	$ 650,492	$ 547,137	$ 554,814	$ 547,483	$ 498,336	$ 2,475,807	$ 2,147,770	$ 1,678,898
Gross profit	103,254	125,901	150,456	140,733	116,793	121,220	131,087	124,014	520,344	493,114	428,903
Income before income taxes	27,572	43,633	62,428	58,719	43,910	47,616	62,626	58,692	192,352	212,844	199,172
Net income	$ 20,179	$ 33,812	$ 47,224	$ 47,336	$ 17,464	$ 32,138	$ 40,137	$ 43,145	$ 148,551	$ 132,884	$ 129,671
Net income per common share:
Basic (in usd per share)	$ 0.80	$ 1.34	$ 1.87	$ 1.88	$ 0.70	$ 1.28	$ 1.61	$ 1.73	$ 5.90	$ 5.31	$ 5.26
Diluted (in usd per share)	0.80	1.33	1.86	1.86	0.68	1.26	1.59	1.71	5.83	5.24	$ 5.20
Stock market price, High (in usd per share)	80.89	102.23	104.90	132.30	132.73	116.63	106.23	117.15	132.30	132.73
Stock market price, Low (in usd per share)	59.68	98.40	80.95	99.46	104.15	93.08	86.25	94.98	59.68	86.25
Close (at end of quarter) (in usd per share)	$ 66.80	$ 69.35	$ 90.15	$ 104.15	$ 130.00	$ 115.85	$ 102.40	$ 99.80	$ 66.80	$ 130.00