Quarterly Financial Information (Unaudited) (Details) - USD ($)	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net Sales	$ 459,570,000	$ 438,628,000	$ 443,139,000	$ 453,535,000	$ 419,913,000	$ 429,643,000	$ 423,801,000	$ 405,568,000	$ 1,794,872,578	$ 1,678,924,756	$ 1,543,617,706
Gross Profit	180,290,000	171,230,000	167,208,000	175,801,000	169,167,000	173,822,000	166,773,000	158,691,000	694,528,266	668,452,244	603,776,052
Operating Income	133,994,000	129,073,000	125,865,000	134,427,000	127,936,000	134,212,000	128,746,000	120,849,000	523,358,352	511,742,935	458,766,439
Net Income	$ 130,469,000	$ 90,230,000	$ 88,536,000	$ 97,557,000	$ 88,761,000	$ 92,065,000	$ 86,485,000	$ 80,280,000	$ 406,791,922	$ 347,591,276	$ 318,469,859
Earnings Per Share (Basic) (in dollars per share)	$ 0.46	$ 0.32	$ 0.31	$ 0.34	$ 0.31	$ 0.32	$ 0.30	$ 0.28	$ 1.42	$ 1.21	$ 1.09
Earnings Per Share (Diluted) (in dollars per share)	$ 0.46	$ 0.31	$ 0.31	$ 0.33	$ 0.31	$ 0.32	$ 0.30	$ 0.28	$ 1.41	$ 1.19	$ 1.08