Consolidated Statements of Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Balance at Dec. 31, 2016	$ 934,541	$ 481	$ 556,663	$ (46)	$ 454,892	$ (77,449)
Balance, shares at Dec. 31, 2016		48,148				11,382
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	169,571				169,571
Issuance of shares pursuant to stock compensation plan	$ 49,845		28,174			$ 21,671
Issuance of shares pursuant to stock compensation plan, shares	1,113					1,113
Stock compensation	$ 37,348		37,348
Issuance of shares pursuant to employee stock purchase plan	$ 7,044		4,682			$ 2,362
Issuance of shares pursuant to employee stock purchase plan, shares	51					51
Treasury stock purchases	$ (6,613)					$ (6,613)
Treasury stock purchases, shares	(44)					(44)
Balance at Dec. 31, 2017	$ 1,191,736	$ 481	626,867	(46)	624,463	$ (60,029)
Balance, shares at Dec. 31, 2017		48,148				10,262
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	147,462				147,462
Issuance of shares pursuant to stock compensation plan	$ 74,907		44,458			$ 30,449
Issuance of shares pursuant to stock compensation plan, shares	1,126					1,126
Stock compensation	$ 52,740		52,740
Issuance of shares pursuant to employee stock purchase plan	$ 8,051		7,370			$ 681
Issuance of shares pursuant to employee stock purchase plan, shares	45					45
Treasury stock purchases	$ (150,050)					$ (150,050)
Treasury stock purchases, shares	(781)					(781)
Balance at Dec. 31, 2018	$ 1,324,846	$ 481	731,435	(46)	771,925	$ (178,949)
Balance, shares at Dec. 31, 2018		48,148				9,872
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	146,527				146,527
Issuance of shares pursuant to stock compensation plan	$ 96,908		(52,833)			$ 149,741
Issuance of shares pursuant to stock compensation plan, shares	999					1,075
Employee taxes paid for withheld shares for taxes upon equity award	$ (5,361)					$ (5,361)
Employee taxes paid for withheld shares for taxes upon equity award, shares						(23)
Stock compensation	59,967		59,967
Issuance of shares pursuant to employee stock purchase plan	$ 9,576		909			$ 8,667
Issuance of shares pursuant to employee stock purchase plan, shares	53					53
Treasury stock purchases	$ (14,289)					$ (14,289)
Treasury stock purchases, shares	(72)					(72)
Balance at Dec. 31, 2019	$ 1,617,058	$ 481	$ 739,478	$ (46)	$ 917,336	$ (40,191)
Balance, shares at Dec. 31, 2019		48,148				8,839