Quarterly Financial Data (Schedule of Quarterly Data) (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
Net sales	$ 226,700	$ 222,100	$ 215,600	$ 199,300	$ 213,000	$ 191,300	$ 203,200	$ 172,300	$ 177,500	$ 178,800	$ 172,300	$ 181,300	$ 863,717	$ 779,839	$ 709,868
Gross profit	77,400	79,400	63,100	75,900	77,400	72,400	78,300	72,500	71,700	75,700	54,600	76,700	295,780	300,568	278,686
Net income/(loss) attributable to the Company	$ 5,900	$ 15,300	$ 1,100	$ 10,800	$ 15,800	$ 13,100	$ 10,400	$ 13,500	$ 37,600	$ 9,700	$ (2,200)	$ 12,200	$ 33,111	$ 52,733	$ 57,279
Basic earnings per share	$ 0.19	$ 0.47	$ 0.03	$ 0.34	$ 0.49	$ 0.41	$ 0.32	$ 0.42	$ 1.18	$ 0.30	$ (0.07)	$ 0.38	$ 1.03	$ 1.64	$ 1.79
Diluted earnings per share	0.19	0.47	0.03	0.34	0.49	0.41	0.32	0.42	1.18	0.30	(0.07)	0.38	1.03	1.64	1.79
Cash dividends per share	0.17	0.17	0.17	0.17	0.17	0.17	0.17	0.17	0.17	0.17	0.17	0.16	$ 0.68	$ 0.68	$ 0.67
Minimum [Member]
Class A Common Stock prices:	56.45	50.25	43.90	43.90	38.65	38.92	37.27	31.43	28.19	28.28	39.15	34.13
Maximum [Member]
Class A Common Stock prices:	$ 65.25	$ 57.60	$ 53.40	$ 49.05	$ 49.25	$ 43.78	$ 41.31	$ 38.21	$ 39.25	$ 40.21	$ 41.15	$ 40.31