CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings	Non-Controlling Interests	Total
BALANCE at Dec. 31, 2014	$ 411	$ (43,598)	$ 320,084	$ 29,384	$ 15,112	$ 321,393
BALANCE (in shares) at Dec. 31, 2014	41,123,365
BALANCE (in shares) at Dec. 31, 2014		3,853,586
Increase (Decrease) in Stockholders' Equity
Net income				49,364	8,076	57,440
Issuance of Stock:
Issuance of shares for options exercised (in shares)		317,333
Issuance of shares for options exercised		$ 3,728	966			4,694
Issuance of restricted stock & performance stock		$ 2,292	(626)			1,666
Issuance of restricted stock & performance stock (in shares)		200,015
Shares received in lieu of tax withholding payment on vested restricted stock		$ (937)				(937)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)		(44,590)
Tax benefit from vesting of restricted stock			284			284
Stock-based compensation			3,057			3,057
Dividends				(9,358)		(9,358)
Distribution to noncontrolling interests					(4,904)	(4,904)
Share repurchase		$ (8,330)				(8,330)
Minimum number of shares of common stock repurchased		(315,953)
BALANCE at Dec. 31, 2015	$ 411	$ (46,845)	323,765	69,390	18,284	365,005
BALANCE (in shares) at Dec. 31, 2015	41,123,365
BALANCE (in shares) at Dec. 31, 2015		3,696,781
Increase (Decrease) in Stockholders' Equity
Net income				64,896		64,896
Issuance of Stock:
Issuance of shares for options exercised		$ 1,568	10			1,578
Issuance of shares for options exercised (in shares)		111,761
Issuance of restricted stock & performance stock		$ 2,282	(306)			1,976
Issuance of restricted stock & performance stock (in shares)		172,727
Shares received in lieu of tax withholding payment on vested restricted stock		$ (1,304)				(1,304)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)		(41,788)
Stock-based compensation			3,502			3,502
Dividends				(10,264)		(10,264)
Share repurchase		$ (13,088)				(13,088)
Minimum number of shares of common stock repurchased		(460,170)
Acquisition of noncontrolling interests			(17,346)		$ (18,284)	(35,630)
BALANCE at Dec. 31, 2016	$ 411	$ (57,387)	309,625	123,984		$ 376,633
BALANCE (in shares) at Dec. 31, 2016	41,123,365					41,123,365
BALANCE (in shares) at Dec. 31, 2016		3,914,251				(3,914,251)
Increase (Decrease) in Stockholders' Equity
Cumulative effect of change in accounting principle (unaudited)				(38)		$ (38)
Net income				55,272		55,272
Issuance of Stock:
Issuance of shares for options exercised		$ 2,257	(205)			2,052
Issuance of shares for options exercised (in shares)		145,746
Issuance of restricted stock & performance stock		$ 2,037	(421)			1,616
Issuance of restricted stock & performance stock (in shares)		134,646
Shares received in lieu of tax withholding payment on vested restricted stock		$ (1,419)				(1,419)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)		(39,335)
Stock-based compensation			3,785			3,785
Dividends				(10,987)		(10,987)
Share repurchase		$ (9,007)				(9,007)
Minimum number of shares of common stock repurchased		(263,097)
BALANCE at Dec. 31, 2017	$ 411	$ (63,519)	$ 312,784	$ 168,269		$ 417,945
BALANCE (in shares) at Dec. 31, 2017	41,123,365					41,123,365
BALANCE (in shares) at Dec. 31, 2017		3,936,291				(3,936,291)