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	37,825				37,825
Issuance of shares pursuant to stock compensation plan	19,298		13,858			$ 5,440
Issuance of shares pursuant to stock compensation plan (in shares)						350
Stock compensation	10,557		10,557
Issuance of shares pursuant to employee stock purchase plan	$ 1,798		1,627			$ 171
Issuance of shares pursuant to employee stock purchase plan (in shares)	12					12
Treasury stock purchases (in shares)	0					0
Treasury stock purchases	$ 0					$ 0
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]
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	27,348				27,348
Issuance of shares pursuant to stock compensation plan	6,528		(14,405)			$ 20,933
Issuance of shares pursuant to stock compensation plan (in shares)						111
Employee taxes paid for withheld shares upon equity award settlement (in shares)						(7)
Employee taxes paid for withheld shares upon equity award settlement	(1,337)					$ (1,337)
Stock compensation	14,416		14,416
Issuance of shares pursuant to employee stock purchase plan	$ 2,349		(373)			$ 2,722
Issuance of shares pursuant to employee stock purchase plan (in shares)	15					15
Treasury stock purchases (in shares)	(72)					(72)
Treasury stock purchases	$ (14,289)					$ (14,289)
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)