Revenue from Contracts with Customers - Impact on Consolidated Income Statement and Consolidated Balance Sheet (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended		6 Months Ended
	Jan. 01, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Assets:
Contract assets	$ 19,125	$ 26,598		$ 26,598		$ 0
Inventories	65,155	78,901		78,901		80,339
Liabilities:
Accounts payable	72,594	74,186		74,186		71,931
Other accrued liabilities	12,899	15,255		15,255		11,690
Deferred tax liabilities	15,185	16,257		16,257		14,714
Stockholders' equity:
Retained earnings	376,381	397,885		397,885		374,783
Income Statement
Net sales		186,413	$ 174,879	378,073	$ 340,366
Cost of sales		150,286	131,552	302,439	256,541
Gross profit		36,127	43,327	75,634	83,825
Selling, product development and administrative expenses		23,878	23,290	49,349	48,640
Operating income		12,249	20,037	26,285	35,185
Interest expense		572	795	1,112	1,401
Other income, net		(368)	792	(53)	1,125
Income before income taxes		12,045	18,450	25,226	32,659
Income tax expense		1,655	5,303	3,778	7,797
Income from equity method investment		(60)	22	(56)	68
Net income		$ 10,450	$ 13,125	$ 21,504	$ 24,794
Earnings per share:
Basic (USD per share)		$ 0.61	$ 0.77	$ 1.25	$ 1.46
Diluted (USD per share)		$ 0.60	$ 0.76	$ 1.24	$ 1.44
Weighted average number of common shares outstanding:
Basic average common shares outstanding		17,196	17,044	17,178	17,014
Diluted average common shares outstanding		17,335	17,262	17,334	17,272
Calculated under Revenue Guidance in Effect before Topic 606
Assets:
Contract assets		$ 0		$ 0		0
Inventories		100,892		100,892		80,339
Liabilities:
Accounts payable		71,295		71,295		71,931
Other accrued liabilities		15,966		15,966		11,690
Deferred tax liabilities		15,729		15,729		14,714
Stockholders' equity:
Retained earnings		395,986		395,986		$ 374,783
Income Statement
Net sales		184,806		370,211
Cost of sales		148,745		294,935
Gross profit		36,061		75,276
Selling, product development and administrative expenses		23,878		49,349
Operating income		12,183		25,927
Interest expense		572		1,112
Other income, net		(368)		(53)
Income before income taxes		11,979		24,868
Income tax expense		1,654		3,721
Income from equity method investment		(60)		(56)
Net income		$ 10,385		$ 21,203
Earnings per share:
Basic (USD per share)		$ 0.60		$ 1.23
Diluted (USD per share)		$ 0.60		$ 1.22
Weighted average number of common shares outstanding:
Basic average common shares outstanding		17,196		17,178
Diluted average common shares outstanding		17,335		17,334
Accounting Standards Update 2014-09
Assets:
Contract assets	7,473
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Assets:
Contract assets	19,125	$ 26,598		$ 26,598
Inventories	(15,184)	(21,991)		(21,991)
Liabilities:
Accounts payable	663	2,891		2,891
Other accrued liabilities	1,209	(711)		(711)
Deferred tax liabilities	471	528		528
Stockholders' equity:
Retained earnings	1,598	1,899		1,899
Income Statement
Net sales	$ 19,600	1,607		7,862
Cost of sales		1,541		7,504
Gross profit		66		358
Selling, product development and administrative expenses		0		0
Operating income		66		358
Interest expense		0		0
Other income, net		0		0
Income before income taxes		66		358
Income tax expense		1		57
Income from equity method investment		0		0
Net income		$ 65		$ 301
Earnings per share:
Basic (USD per share)		$ 0.01		$ 0.02
Diluted (USD per share)		$ 0.00		$ 0.02
Weighted average number of common shares outstanding:
Basic average common shares outstanding		0		0
Diluted average common shares outstanding		0		0