Note 2 - Revenue Recognition - Impact of Topic 606 (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts receivable, net	$ 55,214				$ 38,037				$ 55,214	$ 38,037
Inventories	136,384				99,281				136,384	99,281
Other current assets	11,931				12,762				11,931	12,762
Total current assets	580,810				449,170				580,810	449,170
Deferred tax assets, net	16,830				15,917				16,830	15,917
Total assets	793,432				652,569				793,432	652,569
Accrued liabilities	38,962				27,507				38,962	27,507
Total current liabilities	80,439				65,917				80,439	65,917
Total liabilities	153,339				130,562				153,339	130,562
Retained earnings	194,728				143,608				194,728	143,608
Total stockholders' equity	640,093				522,007				640,093	522,007	$ 431,116	$ 368,516
Total liabilities and stockholders' equity	793,432				652,569				793,432	652,569
Revenue	153,497	$ 159,975	$ 139,761	$ 129,150	129,430	$ 128,939	$ 112,198	$ 100,362	582,382	470,929	388,665
Cost of revenue	68,904	70,957	62,197	57,655	58,269	58,083	50,773	45,520	259,714	212,646	177,792
Gross profit	84,593	89,018	77,564	71,495	71,161	70,856	61,425	54,842	322,668	258,283	210,873
Income from operations	33,077	33,493	24,882	22,037	25,053	23,832	14,970	13,570	113,488	77,424	54,447
Income before income taxes	32,684	36,207	27,114	22,477	26,700	25,087	16,207	14,951	118,482	82,944	57,264
Income tax provision	5,046	4,639	2,908	621	14,629	1,445	1,193	474	13,214	17,741	4,544
Net income	$ 27,638	$ 31,568	$ 24,206	$ 21,856	$ 12,071	$ 23,642	$ 15,014	$ 14,477	$ 105,268	$ 65,203	$ 52,720
Basic (in dollars per share)	$ 0.65	$ 0.75	$ 0.57	$ 0.52	$ 0.29	$ 0.57	$ 0.36	$ 0.35	$ 2.49	$ 1.58	$ 1.30
Comprehensive income									$ 97,912	$ 71,231	$ 47,039
Accounts receivable									(18,079)	(3,785)	(3,421)
Inventories									(37,060)	(27,795)	(8,323)
Other assets									(1,075)	1,603	(11,021)
Accrued liabilities									7,092	10,852	8,035
Income tax liabilities									6,923	$ 29,347	$ 3,165
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Accounts receivable, net	$ 56,502								56,502
Inventories	136,526								136,526
Other current assets	10,329								10,329
Total current assets	580,638								580,638
Deferred tax assets, net	16,931								16,931
Total assets	793,361								793,361
Accrued liabilities	39,103								39,103
Total current liabilities	80,580								80,580
Total liabilities	153,480								153,480
Retained earnings	194,516								194,516
Total stockholders' equity	639,881								639,881
Total liabilities and stockholders' equity	793,361								793,361
Revenue									582,601
Cost of revenue									259,722
Gross profit									322,879
Income from operations									113,699
Income before income taxes									118,693
Income tax provision									13,258
Net income									$ 105,435
Basic (in dollars per share)									$ 2.50
Comprehensive income									$ 98,079
Accounts receivable									(18,465)
Inventories									(37,202)
Other assets									435
Accrued liabilities									5,899
Income tax liabilities									6,967
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Accounts receivable, net	(1,288)								(1,288)
Inventories	(142)								(142)
Other current assets	1,602								1,602
Total current assets	172								172
Deferred tax assets, net	(101)								(101)
Total assets	71								71
Accrued liabilities	(141)								(141)
Total current liabilities	(141)								(141)
Total liabilities	(141)								(141)
Retained earnings	212								212
Total stockholders' equity	212								212
Total liabilities and stockholders' equity	$ 71								71
Revenue									(219)
Cost of revenue									(8)
Gross profit									(211)
Income from operations									(211)
Income before income taxes									(211)
Income tax provision									(44)
Net income									$ (167)
Basic (in dollars per share)									$ (0.01)
Comprehensive income									$ (167)
Accounts receivable									386
Inventories									142
Other assets									(1,510)
Accrued liabilities									1,193
Income tax liabilities									$ (44)