EQUITY (Details) - USD ($)	12 Months Ended			145 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 18, 2016	Sep. 19, 2016	Sep. 30, 2004
Stock-based compensation
Number of shares reserved to be granted under the plan	2,000,000
Treasury Stock
Shares Repurchased	234,746	70,686	37,567
Aggregate Cost	$ 12,983,000	$ 4,873,000	$ 2,705,000
Shares repurchased to satisfy tax withholdings and exercise obligations	32,814	38,279	37,567
Aggregate cost to satisfy tax withholdings and exercise obligations	$ 2,348,000	$ 2,775,000	$ 2,705,000
Average Repurchase Price (in dollars per share)	$ 71.54	$ 72.50	$ 72.01
Number of Options
Exercised (in shares)	(35,081)	(43,053)	(87,378)
Stock-based compensation, additional disclosures
Cash proceeds received upon exercise of options	$ 1,030,000	$ 649,000	$ 1,998,000
2004 Repurchase Plan
Treasury Stock
Authorized amount						$ 5,000,000
Shares Repurchased				4,100,000
2016 Repurchase Plan
Treasury Stock
Authorized amount					$ 50,000,000
Value of shares available for repurchase	$ 39,300,000
2004 and 2016 Repurchase Plan
Treasury Stock
Shares Repurchased	201,932	32,407
Aggregate Cost	$ 10,636,000	$ 2,195,000
Average Repurchase Price (in dollars per share)	$ 52.67	$ 64.72
Stock options
Stock-based compensation
Expiration term	10 years
Vesting period	4 years
Number of Options
Outstanding at the beginning of the period (in shares)	225,822	268,875
Granted (in shares)	10,000
Exercised (in shares)	(35,801)	(43,053)
Outstanding at the end of the period (in shares)	200,021	225,822	268,875
Vested and expected to vest at the end of the period (in shares)	200,021	225,554
Exercisable at the end of the period (in shares)	187,521	222,072
Weighted Avg. Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 39.77	$ 38.64
Granted (in dollars per share)	52.97
Exercised (in dollars per share)	32.29	32.70
Outstanding at the end of the period (in dollars per share)	41.76	39.77	$ 38.64
Vested and expected to vest at the end of the period (in dollars per share)	41.76	39.73
Exercisable at the end of the period (in dollars per share)	$ 40.77	$ 39.23
Weighted Average Remaining Contractual Term
Outstanding	3 years 7 months 6 days	3 years 10 months 24 days
Vested and expected to vest	3 years 7 months 6 days	3 years 10 months 24 days
Exercisable	3 years 7 months 6 days	3 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding	$ 2,780,253	$ 9,114,963
Vested and expected to vest	2,780,253	9,112,630
Exercisable	2,757,353	9,082,338
Unamortized stock based compensation
Unamortized stock based compensation	$ 200,000
Weighted-average period for recognition of unamortized stock-based compensation cost	3 years
Stock-based compensation, additional disclosures
Weighted-average fair value of options granted (in dollars per share)	$ 13.77		$ 30.70
Aggregate intrinsic value of options exercised	$ 936,000	1,591,000	$ 3,488,000
Cash proceeds received upon exercise of options	$ 1,030,000	649,000	1,998,000
Excess tax benefits from share-based compensation		591,000	$ 1,423,000
Weighted-average assumptions using Black-Scholes option-pricing model for estimating fair value of each option granted
Risk-free interest rate (as a percent)	1.79%		1.55%
Expected dividend yield (as a percent)	1.28%		1.76%
Expected life	4 years		6 years 3 months
Expected volatility (as a percent)	34.01%		51.85%
Stock compensation expense
Stock compensation expense	$ 100,000	100,000	$ 400,000
Restricted Stock
Stock-based compensation
Vesting period	4 years
Unamortized stock based compensation
Unamortized stock based compensation	$ 10,200,000
Weighted-average period for recognition of unamortized stock-based compensation cost	2 years 4 months 24 days
Stock compensation expense
Stock compensation expense	$ 6,600,000	6,200,000	4,300,000
Additional compensation expenses	$ 300,000	$ 100,000	$ 300,000
Shares
Unvested at the beginning of the period (in shares)	229,040	218,401
Granted (in shares)	95,095	100,005	93,864
Forfeited (in shares)	(12,074)	(1,125)
Vested and issued (in shares)	(97,123)	(88,241)
Unvested at the end of the period (in shares)	214,938	229,040	218,401
Weighted Avg. Fair Value
Unvested at the beginning of the period (in dollars per share)	$ 67.13	$ 60.60
Granted (in dollars per share)	68.09	73.34
Forfeited (in dollars per share)	69.71	66.44
Vested and issued (in dollars per share)	64.01	58.01
Unvested at the end of the period (in dollars per share)	$ 68.62	$ 67.13	$ 60.60
2008 Plan | Restricted Stock
Stock-based compensation
Vesting period	4 years