Condensed Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss
Beginning Balance (in shares) at Dec. 31, 2017		37,729
Beginning Balance at Dec. 31, 2017	$ 1,191,971	$ 377	$ 266,035	$ 1,045,774	$ (120,215)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	38,945			38,945
Other comprehensive loss:
Cumulative translation adjustment	10,446				10,446
Issuance of common stock in connection with:
Exercise of options (in shares)		153
Exercise of options	5,794	$ 1	5,793
Restricted share grants, less net settled shares (in shares)		175
Restricted share grants, less net settled shares	(1,579)	$ 2	(1,581)
Stock units issued under incentive compensation plan	1,059		1,059
Purchase and retirement of common stock (in shares)		(337)
Purchase and retirement of common stock	(14,220)	$ (3)	(14,217)
Share-based compensation	4,676		4,676
Ending Balance (in shares) at Mar. 31, 2018		37,720
Ending Balance at Mar. 31, 2018	1,237,434	$ 377	261,765	1,085,061	(109,769)
Beginning Balance (in shares) at Dec. 31, 2017		37,729
Beginning Balance at Dec. 31, 2017	1,191,971	$ 377	266,035	1,045,774	(120,215)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	126,887
Other comprehensive loss:
Cumulative translation adjustment	(17,417)
Issuance of common stock in connection with:
Stock units issued under incentive compensation plan	1,059
Ending Balance (in shares) at Sep. 30, 2018		38,356
Ending Balance at Sep. 30, 2018	1,351,475	$ 384	315,720	1,173,003	(137,632)
Beginning Balance (in shares) at Mar. 31, 2018		37,720
Beginning Balance at Mar. 31, 2018	1,237,434	$ 377	261,765	1,085,061	(109,769)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	43,609			43,609
Other comprehensive loss:
Cumulative translation adjustment	(23,683)				(23,683)
Issuance of common stock in connection with:
Exercise of options (in shares)		360
Exercise of options	14,806	$ 4	14,802
Restricted share grants, less net settled shares (in shares)		99
Restricted share grants, less net settled shares	(252)	$ 1	(253)
Share-based compensation	3,887		3,887
Ending Balance (in shares) at Jun. 30, 2018		38,179
Ending Balance at Jun. 30, 2018	1,275,801	$ 382	280,201	1,128,670	(133,452)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	44,333			44,333
Other comprehensive loss:
Cumulative translation adjustment	(4,180)				(4,180)
Issuance of common stock in connection with:
Exercise of options (in shares)		340
Exercise of options	13,569	$ 4	13,565
Restricted share grants, less net settled shares (in shares)		33
Restricted share grants, less net settled shares	(835)	$ 0	(835)
Purchase and retirement of common stock (in shares)		(196)
Purchase and retirement of common stock	(15,000)	$ (2)	(14,998)
Conversion feature of convertible senior notes due 2023, net	34,131		34,131
Share-based compensation	3,656		3,656
Ending Balance (in shares) at Sep. 30, 2018		38,356
Ending Balance at Sep. 30, 2018	$ 1,351,475	$ 384	315,720	1,173,003	(137,632)
Beginning Balance (in shares) at Dec. 31, 2018	38,147	38,147
Beginning Balance at Dec. 31, 2018	$ 1,348,825	$ 381	299,534	1,196,727	(147,817)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	62,645			62,645
Other comprehensive loss:
Cumulative translation adjustment	5,223				5,223
Issuance of common stock in connection with:
Exercise of options (in shares)		55
Exercise of options	2,212	$ 1	2,211
Restricted share grants, less net settled shares (in shares)		153
Restricted share grants, less net settled shares	(2,739)	$ 1	(2,740)
Stock units issued under incentive compensation plan	1,346		1,346
Purchase and retirement of common stock (in shares)		(328)
Purchase and retirement of common stock	(21,883)	$ (3)	(21,880)
Share-based compensation	6,393		6,393
Ending Balance (in shares) at Mar. 31, 2019		38,027
Ending Balance at Mar. 31, 2019	$ 1,402,022	$ 380	284,864	1,259,372	(142,594)
Beginning Balance (in shares) at Dec. 31, 2018	38,147	38,147
Beginning Balance at Dec. 31, 2018	$ 1,348,825	$ 381	299,534	1,196,727	(147,817)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	187,665
Other comprehensive loss:
Cumulative translation adjustment	(16,225)
Issuance of common stock in connection with:
Stock units issued under incentive compensation plan	$ 1,413
Ending Balance (in shares) at Sep. 30, 2019	37,629	37,629
Ending Balance at Sep. 30, 2019	$ 1,461,234	$ 376	240,508	1,384,392	(164,042)
Beginning Balance (in shares) at Mar. 31, 2019		38,027
Beginning Balance at Mar. 31, 2019	1,402,022	$ 380	284,864	1,259,372	(142,594)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	64,598			64,598
Other comprehensive loss:
Cumulative translation adjustment	(4,815)				(4,815)
Issuance of common stock in connection with:
Exercise of options (in shares)		87
Exercise of options	3,076	$ 1	3,075
Restricted share grants, less net settled shares (in shares)		78
Restricted share grants, less net settled shares	(1,351)	$ 1	(1,352)
Purchase and retirement of common stock (in shares)		(580)
Purchase and retirement of common stock	(48,332)	$ (6)	(48,326)
Share-based compensation	3,814		3,814
Ending Balance (in shares) at Jun. 30, 2019		37,612
Ending Balance at Jun. 30, 2019	1,419,012	$ 376	242,075	1,323,970	(147,409)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	60,422			60,422
Other comprehensive loss:
Cumulative translation adjustment	(16,633)				(16,633)
Issuance of common stock in connection with:
Exercise of options (in shares)		94
Exercise of options	3,569	$ 1	3,568
Restricted share grants, less net settled shares (in shares)		14
Restricted share grants, less net settled shares	(1,322)	$ 0	(1,322)
Stock units issued under incentive compensation plan	67		67
Purchase and retirement of common stock (in shares)		(91)
Purchase and retirement of common stock	(7,734)	$ (1)	(7,733)
Share-based compensation	$ 3,853		3,853
Ending Balance (in shares) at Sep. 30, 2019	37,629	37,629
Ending Balance at Sep. 30, 2019	$ 1,461,234	$ 376	$ 240,508	$ 1,384,392	$ (164,042)