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	(3,195)	$ 3		(3,198)
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Shares		288
Stock-based compensation expense	10,764			10,763		1
Repurchase of common stock, Amount	(22,069)		$ (22,069)
Repurchase of common stock, Shares			(641)
Retirement of treasury stock, Amount		$ (6)	$ 22,069	(5,655)		(16,408)
Retirement of treasury stock, Shares		(641)	641
Foreign currency translation adjustments, net of tax	(9,774)				(9,774)
Net earnings	116,636					116,636
Ending Balance at Sep. 30, 2018	943,007	$ 355		319,065	(34,038)	657,625
Ending Balance, Shares at Sep. 30, 2018		35,476
Beginning Balance at Jun. 30, 2018	906,749	$ 355		315,619	(34,695)	625,470
Beginning Balance, Shares at Jun. 30, 2018		35,459
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Value	(270)			(270)
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Shares		17
Stock-based compensation expense	3,717			3,716		1
Foreign currency translation adjustments, net of tax	657				657
Net earnings	32,154					32,154
Ending Balance at Sep. 30, 2018	943,007	$ 355		319,065	(34,038)	657,625
Ending Balance, Shares at Sep. 30, 2018		35,476
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	(6,419)	$ 3		(6,422)
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Shares		310
Stock-based compensation expense	11,895			11,895
Equity component of convertible senior notes, net of deferred tax of $14,819 and issuance costs of $1,700	44,731			44,731
Issuance of warrants related to convertible senior notes	34,440			34,440
Purchase of note hedge related to convertible senior notes, net of deferred tax of $16,047	(50,278)			(50,278)
Repurchase of common stock, Amount	(27,899)		$ (27,899)
Repurchase of common stock, Shares			(541)
Retirement of treasury stock, Amount		$ (5)	$ 27,899	(4,919)		(22,975)
Retirement of treasury stock, Shares		(541)	541
Foreign currency translation adjustments, net of tax	(7,278)				(7,278)
Net earnings	116,457					116,457
Ending Balance at Sep. 30, 2019	1,102,638	$ 353		353,069	(48,931)	798,147
Ending Balance, Shares at Sep. 30, 2019		35,251
Beginning Balance at Jun. 30, 2019	1,079,583	$ 358		325,263	(40,028)	793,990
Beginning Balance, Shares at Jun. 30, 2019		35,781
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Value	(266)			(266)
Issuance of common stock under employee stock plans, net of shares withheld for payroll taxes, Shares		11
Stock-based compensation expense	4,098			4,098
Equity component of convertible senior notes, net of deferred tax of $14,819 and issuance costs of $1,700	44,731			44,731
Issuance of warrants related to convertible senior notes	34,440			34,440
Purchase of note hedge related to convertible senior notes, net of deferred tax of $16,047	(50,278)			(50,278)
Repurchase of common stock, Amount	(27,899)		$ (27,899)
Repurchase of common stock, Shares			(541)
Retirement of treasury stock, Amount		$ (5)	$ 27,899	(4,919)		(22,975)
Retirement of treasury stock, Shares		(541)	541
Foreign currency translation adjustments, net of tax	(8,903)				(8,903)
Net earnings	27,132					27,132
Ending Balance at Sep. 30, 2019	$ 1,102,638	$ 353		$ 353,069	$ (48,931)	$ 798,147
Ending Balance, Shares at Sep. 30, 2019		35,251