SIGNIFICANT ACCOUNTING POLICIES (Details) (USD $)	3 Months Ended						6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements of Income and Comprehensive Income
Net sales	$ 271,053,000	$ 267,798,000	$ 229,350,000	$ 242,536,000	$ 247,902,000	$ 214,355,000	$ 497,148,000	$ 462,257,000	$ 768,201,000	$ 704,794,000	$ 974,429,000	$ 907,794,000	$ 808,766,000
Material costs	79,401,000			73,109,000					226,585,000	212,706,000
Personnel costs	94,768,000			84,135,000					277,897,000	248,723,000
Other operating expenses	47,754,000	49,022,000	39,658,000	41,210,000	47,404,000	36,483,000	88,680,000	83,887,000	136,434,000	125,099,000	177,747,000	167,165,000	156,545,000
Income from operations	39,906,000			35,373,000					99,266,000	92,834,000
Income before taxes	33,889,000			27,622,000					71,119,000	69,622,000
Provision for income taxes	12,230,000			10,278,000					25,891,000	26,257,000
Net income	21,659,000			17,344,000					45,228,000	43,365,000
Basic earnings (in dollars per share)	$ 0.62			$ 0.50					$ 1.30	$ 1.27
Diluted earnings (in dollars per share)	$ 0.61			$ 0.50					$ 1.28	$ 1.25
Consolidated Statements of Cash Flows
Provision for doubtful accounts			1,657,000			907,000	4,591,000	4,462,000	8,545,000	6,580,000	9,589,000	9,396,000	7,252,000
Change in accounts receivable			7,216,000			4,667,000	(7,712,000)	(7,727,000)	(14,586,000)	(10,266,000)	(30,259,000)	(26,583,000)	(18,017,000)
Other operating expense
Consolidated Statements of Cash Flows
Acquisition expenses			100,000			100,000
As Originally Reported
Consolidated Statements of Income and Comprehensive Income
Net sales		273,735,000	233,535,000	243,503,000	251,754,000	218,091,000	507,270,000	469,845,000		713,349,000	985,550,000	918,539,000	817,379,000
Material costs										210,107,000
Personnel costs										251,189,000
Other operating expenses		54,959,000	43,843,000	42,177,000	51,256,000	40,219,000	98,802,000	91,475,000		135,566,000	188,868,000	177,910,000	165,158,000
Income from operations										91,055,000
Income before taxes										67,843,000
Provision for income taxes										25,558,000
Net income										42,285,000
Basic earnings (in dollars per share)										$ 1.24
Diluted earnings (in dollars per share)										$ 1.21
Consolidated Statements of Cash Flows
Provision for doubtful accounts			5,229,000			4,723,000	14,905,000	12,925,000		15,926,000	19,773,000	24,837,000	20,276,000
Change in accounts receivable			3,644,000			851,000	(18,026,000)	(16,190,000)		(19,612,000)	(40,443,000)	(42,024,000)	(31,041,000)
As Originally Reported | Other operating expense
Consolidated Statements of Cash Flows
Acquisition expenses			$ 100,000			$ 100,000