Consolidated Statements of Stockholders' Equity (unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Retained Earnings [Member]
Beginning Balance at Dec. 31, 2017	$ 843,469	$ 358		$ 317,155	$ (24,264)	$ 550,220
Beginning Balance, Shares at Dec. 31, 2017		35,829
Cumulative effect of accounting change	7,176					7,176
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Value	(2,885)	$ 2		(2,887)
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Shares		240
Stock-based compensation expense	3,184			3,184
Repurchase of treasury stock, Amount	(7,679)		$ (7,679)
Repurchase of treasury stock, Shares			(221)
Retirement of treasury stock, Amount		$ (2)	$ 7,679	(1,959)		(5,718)
Retirement of treasury stock, Shares		(221)	221
Foreign currency translation adjustments, net of tax	4,591				4,591
Net earnings	33,003					33,003
Ending Balance at Mar. 31, 2018	880,859	$ 358		315,493	(19,673)	584,681
Ending Balance, Shares at Mar. 31, 2018		35,848
Beginning Balance at Dec. 31, 2018	986,989	$ 355		323,622	(41,653)	704,665
Beginning Balance, Shares at Dec. 31, 2018		35,482
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Value	(6,128)	$ 3		(6,131)
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Shares		294
Stock-based compensation expense	4,115			4,115
Retirement of treasury stock, Shares		(641)	641
Foreign currency translation adjustments, net of tax	1,915				1,915
Net earnings	39,327					39,327
Ending Balance at Mar. 31, 2019	$ 1,026,218	$ 358		$ 321,606	$ (39,738)	$ 743,992
Ending Balance, Shares at Mar. 31, 2019		35,761