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	115,910
Exercise of stock options and vesting of restricted stock units (in shares)						1,048
Exercise of stock options and vesting of restricted stock units	70,536		54,666			$ 15,870
Stock compensation	$ 37,966		37,966
Issuance of shares pursuant to employee stock purchase plan (in shares)	35					35
Issuance of shares pursuant to employee stock purchase plan	$ 5,978		5,436			$ 542
Treasury stock purchases (in shares)	0
Treasury stock purchases	$ 0
Balance, shares at Sep. 30, 2018		48,148				9,179
Balance at Sep. 30, 2018	1,422,126	$ 481	724,935	(46)	740,373	$ (43,617)
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]
Net income	38,924				38,924
Exercise of stock options and vesting of restricted stock units (in shares)						326
Exercise of stock options and vesting of restricted stock units	26,219		21,645			$ 4,574
Stock compensation	$ 14,476		14,476
Issuance of shares pursuant to employee stock purchase plan (in shares)	12					12
Issuance of shares pursuant to employee stock purchase plan	$ 2,218		2,032			$ 186
Treasury stock purchases (in shares)	0
Treasury stock purchases	$ 0
Balance, shares at Sep. 30, 2018		48,148				9,179
Balance at Sep. 30, 2018	1,422,126	$ 481	724,935	(46)	740,373	$ (43,617)
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	99,737				99,737
Exercise of stock options and vesting of restricted stock units (in shares)						746
Exercise of stock options and vesting of restricted stock units	62,295		(82,449)			$ 144,744
Employee taxes paid for withheld shares upon equity award settlement (in shares)						(17)
Employee taxes paid for withheld shares upon equity award settlement	(3,572)					$ (3,572)
Stock compensation	$ 44,369		44,369
Issuance of shares pursuant to employee stock purchase plan (in shares)	43					43
Issuance of shares pursuant to employee stock purchase plan	$ 7,327		(769)			$ 8,096
Treasury stock purchases (in shares)	(72)					(72)
Treasury stock purchases	$ (14,289)					$ (14,289)
Balance, shares at Sep. 30, 2019		48,148				9,172
Balance at Sep. 30, 2019	1,519,597	$ 481	692,586	(46)	870,546	$ (43,970)
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	40,390
Exercise of stock options and vesting of restricted stock units (in shares)						396
Exercise of stock options and vesting of restricted stock units	40,163		(38,160)			$ 78,323
Employee taxes paid for withheld shares upon equity award settlement (in shares)						(1)
Employee taxes paid for withheld shares upon equity award settlement	(191)					$ (191)
Stock compensation	$ 14,887		14,887
Issuance of shares pursuant to employee stock purchase plan (in shares)	15					15
Issuance of shares pursuant to employee stock purchase plan	$ 2,718		(61)			$ 2,779
Treasury stock purchases (in shares)	0
Treasury stock purchases	$ 0
Balance, shares at Sep. 30, 2019		48,148				9,172
Balance at Sep. 30, 2019	$ 1,519,597	$ 481	$ 692,586	$ (46)	$ 870,546	$ (43,970)