CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Other Comprehensive Income	Retained Earnings	Noncontrolling Interests
Balance, Stockholders Equity at Dec. 31, 2017	$ 516,426	$ 35	$ 568,780	$ (53,713)	$ 15	$ 204	$ 1,105
Balance (in shares) at Dec. 31, 2017		35,747,134
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock - employee stock purchase plan	2,429		2,429
Issuance of stock - employee stock purchase plan (shares)		38,961
Issuance of stock - 401 (k) plan	9,232		9,232
Issuance of stock - 401 (k) plan (shares)		129,451
Issuance/(cancellation) of non-vested stock	0	$ 1	(1)
Issuance/(cancellation) of non-vested stock (shares)		174,044
Exercise of stock options	5,953		5,953
Exercise of stock options (in shares)		162,690
Non-cash compensation	17,887		17,887
Surrendered shares	(6,570)			(6,570)
Shares repurchased	(181,402)			(181,402)
Noncontrolling interest distribution	(1,090)						(1,090)
Repurchase of noncontrolling interest	(361)		(614)				253
Net income	120,129					119,346	783
Balance, Stockholders Equity at Dec. 31, 2018	482,633	$ 36	603,666	(241,685)	15	119,550	1,051
Balance (in shares) at Dec. 31, 2018		36,252,280
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock - employee stock purchase plan	3,187		3,187
Issuance of stock - employee stock purchase plan (shares)		30,483
Issuance of stock - 401 (k) plan	9,753		9,753
Issuance of stock - 401 (k) plan (shares)		79,056
Issuance/(cancellation) of non-vested stock	0	$ 1	(1)
Issuance/(cancellation) of non-vested stock (shares)		189,134
Exercise of stock options	3,611		3,611
Exercise of stock options (in shares)		87,068
Non-cash compensation	25,040		25,040
Surrendered shares	(9,556)			(9,556)
Noncontrolling interest distribution	(1,062)						(1,062)
Net income	127,907					126,833	1,074
Balance, Stockholders Equity at Dec. 31, 2019	641,513	$ 37	645,256	(251,241)	15	246,383	1,063
Balance (in shares) at Dec. 31, 2019		36,638,021
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock - employee stock purchase plan	3,562		3,562
Issuance of stock - employee stock purchase plan (shares)		21,561
Issuance of stock - 401 (k) plan	3,057		3,057
Issuance of stock - 401 (k) plan (shares)		18,312
Issuance/(cancellation) of non-vested stock	0
Issuance/(cancellation) of non-vested stock (shares)		169,489
Exercise of stock options	$ 6,325	$ 1	6,324
Exercise of stock options (in shares)	622,829	622,829
Non-cash compensation	$ 26,730		26,730
Surrendered shares	(54,493)		13,358	(67,851)
Noncontrolling interest distribution	(1,122)						(1,122)
Write-off of other comprehensive income	(15)				(15)
Net income	185,184					183,608	1,576
Balance, Stockholders Equity at Dec. 31, 2020	$ 810,741	$ 38	$ 698,287	$ (319,092)	$ 0	$ 429,991	$ 1,517
Balance (in shares) at Dec. 31, 2020		37,470,212