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, shares at Dec. 31, 2017		48,148				10,262
Balance at Dec. 31, 2017	$ 1,191,736	$ 481	$ 626,867	$ (46)	$ 624,463	$ (60,029)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	76,986
Exercise of stock options and vesting of restricted stock units (in shares)						722
Exercise of stock options and vesting of restricted stock units	44,317		33,021			$ 11,296
Stock compensation	$ 23,490		23,490
Issuance of shares pursuant to employee stock purchase plan (in shares)	23					23
Issuance of shares pursuant to employee stock purchase plan	$ 3,760		3,404			$ 356
Treasury stock purchases (in shares)	0					0
Treasury stock purchases	$ 0					$ 0
Balance, shares at Jun. 30, 2018		48,148				9,517
Balance at Jun. 30, 2018	1,340,289	$ 481	686,782	(46)	701,449	$ (48,377)
Balance, shares at Mar. 31, 2018		48,148				9,900
Balance at Mar. 31, 2018	1,261,214	$ 481	652,909	(46)	662,288	$ (54,418)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	39,161				39,161
Exercise of stock options and vesting of restricted stock units (in shares)						372
Exercise of stock options and vesting of restricted stock units	25,019		19,163			$ 5,856
Stock compensation	$ 12,933		12,933
Issuance of shares pursuant to employee stock purchase plan (in shares)	11					11
Issuance of shares pursuant to employee stock purchase plan	$ 1,962		1,777			$ 185
Treasury stock purchases (in shares)	0					0
Treasury stock purchases	$ 0					$ 0
Balance, shares at Jun. 30, 2018		48,148				9,517
Balance at Jun. 30, 2018	1,340,289	$ 481	686,782	(46)	701,449	$ (48,377)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Retained earnings adjustment-adoption of Topic 842 Leases, net of taxes	(1,116)				(1,116)
Balance, shares at Dec. 31, 2018		48,148				9,872
Balance at Dec. 31, 2018	1,324,846	$ 481	731,435	(46)	771,925	$ (178,949)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	59,347				59,347
Exercise of stock options and vesting of restricted stock units (in shares)						350
Exercise of stock options and vesting of restricted stock units	22,132		(44,289)			$ 66,421
Employee taxes paid for withheld shares upon equity award settlement (in shares)						(16)
Employee taxes paid for withheld shares upon equity award settlement	(3,381)					$ (3,381)
Stock compensation	$ 29,482		29,482
Issuance of shares pursuant to employee stock purchase plan (in shares)	28					28
Issuance of shares pursuant to employee stock purchase plan	$ 4,609		(708)			$ 5,317
Treasury stock purchases (in shares)	(72)					(72)
Treasury stock purchases	$ (14,289)					$ (14,289)
Balance, shares at Jun. 30, 2019		48,148				9,582
Balance at Jun. 30, 2019	1,421,630	$ 481	715,920	(46)	830,156	$ (124,881)
Balance, shares at Mar. 31, 2019		48,148				9,825
Balance at Mar. 31, 2019	1,358,745	$ 481	731,073	(46)	798,157	$ (170,920)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	31,999
Exercise of stock options and vesting of restricted stock units (in shares)						239
Exercise of stock options and vesting of restricted stock units	15,604		(29,884)			$ 45,488
Employee taxes paid for withheld shares upon equity award settlement (in shares)						(9)
Employee taxes paid for withheld shares upon equity award settlement	(2,044)					$ (2,044)
Stock compensation	$ 15,066		15,066
Issuance of shares pursuant to employee stock purchase plan (in shares)	13					13
Issuance of shares pursuant to employee stock purchase plan	$ 2,260		(335)			$ 2,595
Treasury stock purchases (in shares)	0					0
Treasury stock purchases	$ 0					$ 0
Balance, shares at Jun. 30, 2019		48,148				9,582
Balance at Jun. 30, 2019	$ 1,421,630	$ 481	$ 715,920	$ (46)	$ 830,156	$ (124,881)