SELECTED QUARTERLY FINANCIAL DATA (Detail) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	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, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2008
Selected Quarterly Financial Information [Line Items]
Revenue	$ 217,333,427	$ 219,359,379	$ 245,804,613	$ 218,229,539	$ 254,879,358	$ 238,793,030	$ 191,007,886	$ 169,086,267	$ 177,579,159	$ 155,570,657	$ 167,012,527	$ 136,931,096	$ 900,726,958	$ 853,766,541	$ 637,093,439
Cost of goods sold	(196,857,791)	(198,261,030)	(222,034,732)	(197,174,187)	(229,933,412)	(213,536,628)	(170,296,852)	(150,451,275)	(157,877,953)	(136,113,727)	(147,365,064)	(120,023,023)	(814,327,740)	(764,218,167)	(561,379,767)
Gross profit	20,475,636	21,098,349	23,769,881	21,055,352	24,945,946	25,256,402	20,711,034	18,634,992	19,701,206	19,456,930	19,647,463	16,908,073	86,399,218	89,548,374	75,713,672
Selling expenses	(741,114)	(796,945)	(801,861)	(680,680)	(786,331)	(741,135)	(740,988)	(686,006)	(660,992)	(633,584)	(668,366)	(523,036)	(3,020,600)	(2,954,460)	(2,485,978)
General and administrative expenses	(1,853,646)	(1,807,615)	(2,006,127)	(2,405,784)	(2,630,168)	(2,059,027)	(1,834,465)	(2,097,834)	(2,239,462)	(1,498,306)	(1,339,062)	(1,573,312)	(8,073,172)	(8,621,494)	(6,650,142)
Income from operations	17,880,876	18,493,789	20,961,893	17,968,888	21,529,447	22,456,240	18,135,581	15,851,152	16,800,752	17,325,040	17,640,035	14,811,725	75,305,446	77,972,420	66,577,552
Net income	$ 13,233,558	$ 13,938,895	$ 15,675,810	$ 13,311,187	$ 15,793,687	$ 17,226,638	$ 13,446,666	$ 11,475,012	$ 12,727,388	$ 13,528,083	$ 14,366,076	$ 12,511,986	$ 56,159,450	$ 57,942,003	$ 53,133,533	$ 11,701,879
Earnings per share
Basic	$ 0.44	$ 0.47	$ 0.53	$ 0.45	$ 0.52	$ 0.58	$ 0.45	$ 0.39	$ 0.43	$ 0.45	$ 0.48	$ 0.42	$ 1.88	$ 1.94	$ 1.78
Diluted	$ 0.44	$ 0.46	$ 0.52	$ 0.44	$ 0.52	$ 0.58	$ 0.45	$ 0.38	$ 0.43	$ 0.45	$ 0.48	$ 0.41	$ 1.87	$ 1.93	$ 1.77