Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance (in shares) at Dec. 31, 2017	26,828,651
Balance at Dec. 31, 2017	$ 27	$ 241,725	$ 224,697	$ (5,234)	$ 461,215
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	102,895
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		2,250			2,250
Stock-based compensation expense		2,307			2,307
Repurchases of common stock (in shares)	0
Net income			18,051		18,051
Other comprehensive income
Foreign currency translation adjustment				2,393	2,393
Comprehensive income					20,444
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	102,895
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		2,250			2,250
Stock-based compensation expense		2,307			2,307
Repurchases of common stock (in shares)	0
Net income			18,051		18,051
Balance (in shares) at Dec. 31, 2017	26,828,651
Balance at Dec. 31, 2017	$ 27	241,725	224,697	(5,234)	461,215
Balance (in shares) at Mar. 31, 2018	26,931,546
Balance at Mar. 31, 2018	$ 27	246,282	244,208	(2,841)	487,676
Balance (in shares) at Dec. 31, 2017	26,828,651
Balance at Dec. 31, 2017	$ 27	241,725	224,697	(5,234)	461,215
Net income					57,302
Other comprehensive income
Comprehensive income					55,298
Net income					57,302
Balance (in shares) at Dec. 31, 2017	26,828,651
Balance at Dec. 31, 2017	$ 27	241,725	224,697	(5,234)	461,215
Balance (in shares) at Sep. 30, 2018	27,068,068
Balance at Sep. 30, 2018	$ 27	254,958	283,459	(7,238)	531,206
Other comprehensive income
Revenue recognition transition adjustment			1,460		1,460
Balance (in shares) at Mar. 31, 2018	26,931,546
Balance at Mar. 31, 2018	$ 27	246,282	244,208	(2,841)	487,676
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	94,848
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		1,054			1,054
Stock-based compensation expense		2,727			2,727
Repurchases of common stock (in shares)	0
Net income			18,314		18,314
Other comprehensive income
Foreign currency translation adjustment				(3,518)	(3,518)
Comprehensive income					14,796
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	94,848
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		1,054			1,054
Stock-based compensation expense		2,727			2,727
Repurchases of common stock (in shares)	0
Net income			18,314		18,314
Balance (in shares) at Mar. 31, 2018	26,931,546
Balance at Mar. 31, 2018	$ 27	246,282	244,208	(2,841)	487,676
Balance (in shares) at Jun. 30, 2018	27,026,394
Balance at Jun. 30, 2018	$ 27	250,063	262,522	(6,359)	506,253
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	41,674
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		1,942			1,942
Stock-based compensation expense		2,953			2,953
Repurchases of common stock (in shares)	0
Net income			20,937		20,937
Other comprehensive income
Foreign currency translation adjustment				(879)	(879)
Comprehensive income					20,058
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	41,674
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		1,942			1,942
Stock-based compensation expense		2,953			2,953
Repurchases of common stock (in shares)	0
Net income			20,937		20,937
Balance (in shares) at Jun. 30, 2018	27,026,394
Balance at Jun. 30, 2018	$ 27	250,063	262,522	(6,359)	506,253
Balance (in shares) at Sep. 30, 2018	27,068,068
Balance at Sep. 30, 2018	$ 27	254,958	283,459	(7,238)	$ 531,206
Balance (in shares) at Dec. 31, 2018	26,984,747				26,984,747
Balance at Dec. 31, 2018	$ 27	258,502	291,460	(8,492)	$ 541,497
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	19,950
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		(420)			(420)
Stock-based compensation expense		3,040			3,040
Repurchases of common stock (in shares)	(157,716)
Repurchases of common stock		(1,511)	(15,798)		(17,309)
Net income			15,511		15,511
Other comprehensive income
Foreign currency translation adjustment				476	476
Comprehensive income					15,987
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	19,950
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		(420)			(420)
Stock-based compensation expense		3,040			3,040
Repurchases of common stock (in shares)	(157,716)
Repurchases of common stock		(1,511)	(15,798)		(17,309)
Net income			15,511		$ 15,511
Balance (in shares) at Dec. 31, 2018	26,984,747				26,984,747
Balance at Dec. 31, 2018	$ 27	258,502	291,460	(8,492)	$ 541,497
Balance (in shares) at Mar. 31, 2019	26,846,981
Balance at Mar. 31, 2019	$ 27	259,611	291,173	(8,016)	$ 542,795
Balance (in shares) at Dec. 31, 2018	26,984,747				26,984,747
Balance at Dec. 31, 2018	$ 27	258,502	291,460	(8,492)	$ 541,497
Net income					48,469
Other comprehensive income
Comprehensive income					46,317
Net income					$ 48,469
Balance (in shares) at Dec. 31, 2018	26,984,747				26,984,747
Balance at Dec. 31, 2018	$ 27	258,502	291,460	(8,492)	$ 541,497
Balance (in shares) at Sep. 30, 2019	26,759,109				26,759,109
Balance at Sep. 30, 2019	$ 27	265,178	309,536	(10,644)	$ 564,097
Balance (in shares) at Mar. 31, 2019	26,846,981
Balance at Mar. 31, 2019	$ 27	259,611	291,173	(8,016)	542,795
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	75,785
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		245			245
Stock-based compensation expense		3,486			3,486
Repurchases of common stock (in shares)	(40,669)
Repurchases of common stock		(390)	(3,711)		(4,101)
Net income			16,166		16,166
Other comprehensive income
Foreign currency translation adjustment				(478)	(478)
Comprehensive income					15,688
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	75,785
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		245			245
Stock-based compensation expense		3,486			3,486
Repurchases of common stock (in shares)	(40,669)
Repurchases of common stock		(390)	(3,711)		(4,101)
Net income			16,166		16,166
Balance (in shares) at Mar. 31, 2019	26,846,981
Balance at Mar. 31, 2019	$ 27	259,611	291,173	(8,016)	542,795
Balance (in shares) at Jun. 30, 2019	26,882,097
Balance at Jun. 30, 2019	$ 27	262,952	303,628	(8,494)	558,113
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	5,089
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		241			241
Stock-based compensation expense		3,212			3,212
Repurchases of common stock (in shares)	(128,077)
Repurchases of common stock		(1,227)	(10,884)		(12,111)
Net income			16,792		16,792
Other comprehensive income
Foreign currency translation adjustment				(2,150)	(2,150)
Comprehensive income					14,642
Common shares issued on exercise of options and other, net of shares withheld for tax obligations (in shares)	5,089
Common shares issued on exercise of options and other, net of shares withheld for tax obligations		241			241
Stock-based compensation expense		3,212			3,212
Repurchases of common stock (in shares)	(128,077)
Repurchases of common stock		(1,227)	(10,884)		(12,111)
Net income			16,792		16,792
Balance (in shares) at Jun. 30, 2019	26,882,097
Balance at Jun. 30, 2019	$ 27	262,952	303,628	(8,494)	$ 558,113
Balance (in shares) at Sep. 30, 2019	26,759,109				26,759,109
Balance at Sep. 30, 2019	$ 27	$ 265,178	$ 309,536	$ (10,644)	$ 564,097