Quarterly Financial Information (Unaudited) (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Net Sales	$ 260,349,000		$ 268,248,000		$ 280,256,000		$ 290,707,000		$ 260,347,000		$ 269,468,000		$ 243,002,000		$ 250,946,000		$ 1,099,559,521	$ 1,023,762,049	$ 816,263,414
Gross Profit	89,119,000		90,116,000		92,757,000		100,826,000		90,354,000		95,285,000		85,625,000		90,317,000		372,818,559	361,580,335	295,690,313
Operating Income	53,819,000		57,623,000		57,512,000		65,501,000		55,783,000		62,247,000		53,247,000		60,091,000		234,455,232	231,367,925	190,972,069
Net Income	$ 39,599,000		$ 41,877,000		$ 40,767,000		$ 46,345,000		$ 40,466,000		$ 43,397,000		$ 38,472,000		$ 42,333,000		$ 168,586,840	$ 164,668,228	$ 137,733,969
Diluted earnings per share (usd per share)	$ 0.28	[1]	$ 0.29	[1]	$ 0.28	[1]	$ 0.32	[1]	$ 0.28	[1]	$ 0.30	[1]	$ 0.27	[1]	$ 0.29	[1]	$ 1.17	$ 1.14	$ 0.98

[1] *Diluted